diff --git a/docs/user-flows.md b/docs/user-flows.md
index 5f43767..d2eb3f2 100644
--- a/docs/user-flows.md
+++ b/docs/user-flows.md
@@ -239,7 +239,12 @@ the destination is reachable, with its calendar open.
   with driving alone and flying, from ADEME factors over the straight-line distance
   (`src/util/eco.ts`).
 - **Settings** — theme, MAX Jeune/Senior, comfortable/compact, and Low-end mode (map off +
-  reduced motion + compact) with a one-time nudge on weak devices; language.
+  reduced motion + compact) with a one-time nudge on weak devices; language. "Interrail
+  reservation fees" (off by default) adds each trip's pass-holder reservation fee, 2nd class,
+  to the Interrail exact-trip list (connections add up their trains), a "Cheapest
+  reservation" sort and a note linking Interrail's fee page. The fees come from
+  `src/data/fees.ts`, the same table train-api publishes; a train with no published fee
+  (SNCF coaches) shows none and sorts last.
 - **Mobile** — the form is a sheet that collapses to a search bar (a long route wraps it to
   two lines); results are a bottom-sheet drawer with peek / half / full detents measured
   below that bar, so the full sheet never covers it. Back navigation preserves form state
diff --git a/src/app.ts b/src/app.ts
index ca99433..2cd577a 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -27,6 +27,7 @@ import { findHiddenTrains } from "./core/hidden";
 import { addDays, dayIndex, formatDuration, minutesToHHMM } from "./util/time";
 import { haversineKm } from "./util/geo";
 import { tripCo2 } from "./util/eco";
+import { journeyFee, type Fee } from "./data/fees";
 import { el, clear, isTouch } from "./ui/dom";
 import { buildShell, applyTheme, applyDensity, applyReduceMotion, applyMap, closeHeaderMenu } from "./ui/shell";
 import { showPostcard } from "./ui/toast";
@@ -53,6 +54,7 @@ import {
   MAX_SENIOR_URL,
   INTERRAIL_FRANCE_URL,
   INTERRAIL_PASS_URL,
+  INTERRAIL_FEES_URL,
   ADEME_CO2_URL,
   GITHUB_URL,
   GITHUB_ISSUES_URL,
@@ -929,6 +931,8 @@ function cityInfoUrl(id: string): string {
 function ctx(): RenderCtx {
   return {
     label: (id) => deps.registry.label(id),
+    // Interrail reservation fees: an opt-in setting, and only for the Interrail pass.
+    fee: settings.fees && !deps.profile.seatKnown ? journeyFee : undefined,
     formatDate,
     formatWeekday,
     // Deep-link to SNCF Connect with the trip pre-filled (clean station names, the
@@ -1730,6 +1734,7 @@ const SORT_LABEL = {
   name: "sort_name",
   arrival: "sort_arrival",
   departure: "sort_departure",
+  cheapest: "sort_cheapest",
 } as const;
 
 /** Build the sort-picker option list for a mode from its applicable keys. */
@@ -1751,6 +1756,7 @@ interface SortAccessors<T> {
   durationMin?: (x: T) => number;
   arriveMin?: (x: T) => number; // absolute arrival minutes (earliest first)
   departMin?: (x: T) => number; // departure minutes (earliest first)
+  fee?: (x: T) => Fee | undefined; // Interrail reservation fees (cheapest first, unknown last)
 }
 
 /**
@@ -1784,6 +1790,14 @@ function applySort<T>(items: T[], acc: SortAccessors<T>): T[] {
     case "name":
       arr.sort((a, b) => acc.name(a).localeCompare(acc.name(b)));
       break;
+    case "cheapest":
+      if (acc.fee) {
+        const fee = acc.fee;
+        const lo = (x: T): number => fee(x)?.min ?? Infinity;
+        const hi = (x: T): number => fee(x)?.max ?? Infinity;
+        arr.sort((a, b) => lo(a) - lo(b) || hi(a) - hi(b));
+      }
+      break;
   }
   return arr;
 }
@@ -2690,6 +2704,18 @@ function appendNearbyAlternatives(
  * journey list booked on click, hidden-city trains and radius alternatives. Round /
  * day trips take the separate {@link runTripSearch} 2-leg accordion instead.
  */
+/** Under an exact trip's list when fees are on: they are indicative, with their source. */
+function feeNoteEl(): HTMLElement {
+  return el("p", { class: "muted small fee-note" }, [
+    el("span", { text: `${t("fee_note")} ` }),
+    el("a", {
+      href: INTERRAIL_FEES_URL,
+      text: t("fee_link"),
+      attrs: { target: "_blank", rel: "noopener noreferrer" },
+    }),
+  ]);
+}
+
 function runOdSearch(c: RenderCtx): void {
   const { trains, registry } = deps;
   if (!query.origin || !query.destination) {
@@ -2761,6 +2787,7 @@ function runOdSearch(c: RenderCtx): void {
       durationMin: (j) => j.totalDurationMin,
       arriveMin: (j) => (flex > 0 ? dayIndex(j.date) * 1440 : 0) + journeyArriveAbs(j),
       departMin: (j) => (flex > 0 ? dayIndex(j.date) * 1440 : 0) + j.departMin,
+      fee: c.fee,
     },
   );
 
@@ -2786,11 +2813,12 @@ function runOdSearch(c: RenderCtx): void {
     refs.results.append(
       render.listToolbarEl(
         t("badge_trains", { n: journeys.length }),
-        query.sort ?? "rec",
-        sortOptions(["rec", "arrival", "departure", "fastest"]),
+        query.sort === "cheapest" && !c.fee ? "rec" : query.sort ?? "rec",
+        sortOptions(c.fee ? ["rec", "arrival", "departure", "fastest", "cheapest"] : ["rec", "arrival", "departure", "fastest"]),
         onSort,
       ),
     );
+    if (c.fee) refs.results.append(feeNoteEl());
     for (const j of journeys)
       refs.results.append(
         render.journeyEl(j, c, { bookOnClick: true, dateLabel: flex > 0 ? formatDate(j.date) : undefined }),
@@ -3530,6 +3558,7 @@ function openSettings(): void {
     reduceMotion: settings.reduceMotion,
     map: settings.map,
     compact: settings.density === "compact",
+    fees: settings.fees,
     onReduceMotion: (v) => {
       settings = { ...settings, reduceMotion: v };
       store.saveSettings(settings);
@@ -3549,6 +3578,11 @@ function openSettings(): void {
       store.saveSettings(settings);
       applyDensity(density);
     },
+    onFees: (v) => {
+      settings = { ...settings, fees: v };
+      store.saveSettings(settings);
+      refreshInPlace();
+    },
     // Master "Low-end mode": flip all three savers together. ON = the light experience
     // (no motion, no map, compact); OFF = the full experience (motion, map, comfortable).
     onLowEnd: (v) => {
diff --git a/src/config.ts b/src/config.ts
index f3bdb91..25eb2f3 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -82,6 +82,9 @@ export const MAX_SENIOR_URL = "https://www.sncf-connect.com/catalogue/descriptio
 export const INTERRAIL_FRANCE_URL = "https://www.interrail.com/en/plan-your-trip/reservations/reservations-france";
 /** Interrail Global Pass conditions: who can buy it, and the country-of-residence rule. */
 export const INTERRAIL_PASS_URL = "https://www.interrail.com/en/interrail-passes/global-pass";
+/** Interrail's pass-holder reservation fees, the source of src/data/fees.ts. */
+export const INTERRAIL_FEES_URL =
+  "https://www.interrail.com/en/book-reservations/reservation-fees/domestic-train-reservation-fees";
 /** ADEME's CO₂ comparator, the source of the per-km factors in src/util/eco.ts. */
 export const ADEME_CO2_URL = "https://impactco2.fr/outils/transport";
 
diff --git a/src/data/fees.ts b/src/data/fees.ts
new file mode 100644
index 0000000..d7ac8fb
--- /dev/null
+++ b/src/data/fees.ts
@@ -0,0 +1,93 @@
+import type { Journey } from "../types";
+
+/**
+ * What an Interrail pass holder pays to reserve a seat on SNCF trains, in euros, 2nd
+ * class. The same table train-api publishes as v1/sncf/interrail/fees.json (see its
+ * src/operators/interrail-fees.ts), hand-entered from Interrail's fee pages and checked
+ * on 2026-10-12. Indicative: the app says so and links the source.
+ *
+ * A stop abroad sets the international fee for that country; otherwise the train's axe
+ * sets the domestic one.
+ */
+
+/** A pass-holder reservation in euros; `min === max` for a fixed fee. */
+export interface Fee {
+  min: number;
+  max: number;
+}
+
+type Country = "BE" | "CH" | "DE" | "ES" | "IT" | "LU";
+
+/** Whole words of SNCF stop names abroad, by country (accents and case ignored). */
+const FOREIGN_STOPS: Record<Country, string[]> = {
+  BE: ["bruxelles", "brussel", "liege", "anvers", "antwerpen"],
+  CH: ["geneve", "lausanne", "zurich", "basel", "bale", "bern", "vallorbe", "neuchatel"],
+  DE: [
+    "mannheim", "saarbrucken", "saarbruecken", "kaiserslautern", "frankfurt", "karlsruhe",
+    "stuttgart", "augsburg", "munchen", "baden baden", "ulm", "freiburg", "koln", "dortmund",
+  ],
+  ES: ["barcelona", "girona", "figueres", "madrid"],
+  IT: ["torino", "milano", "oulx", "oux"],
+  LU: ["luxembourg"],
+};
+
+const BY_COUNTRY: Record<Exclude<Country, "ES">, Fee> = {
+  BE: { min: 20, max: 35 }, // TGV to Brussels
+  CH: { min: 25, max: 25 }, // TGV Lyria
+  DE: { min: 19, max: 19 }, // ICE & TGV France–Germany
+  IT: { min: 31, max: 31 }, // TGV France–Italy
+  LU: { min: 10, max: 20 }, // TGV to Luxembourg
+};
+const SPAIN_FROM_PARIS: Fee = { min: 35, max: 35 };
+const SPAIN_OTHER: Fee = { min: 25, max: 25 };
+
+const BY_AXE: Record<string, Fee> = {
+  "SUD EST": { min: 10, max: 20 }, // TGV INOUI
+  EST: { min: 10, max: 20 },
+  ATLANTIQUE: { min: 10, max: 20 },
+  NORD: { min: 10, max: 20 },
+  INTERNATIONAL: { min: 10, max: 20 }, // a stretch inside France
+  "IC SRO": { min: 10, max: 20 }, // Intercités, reservation compulsory
+  "IC ARO": { min: 10, max: 20 },
+  "IC NUIT": { min: 10, max: 25 }, // seat €10, couchette €19–25
+};
+
+function fold(s: string): string {
+  return ` ${s.normalize("NFD").replace(/[̀-ͯ]/g, "").toLowerCase().replace(/[^a-z0-9]+/g, " ")} `;
+}
+
+function foreignCountry(stop: string): Country | undefined {
+  const f = fold(stop);
+  for (const [country, names] of Object.entries(FOREIGN_STOPS) as [Country, string[]][]) {
+    if (names.some((n) => f.includes(` ${n} `))) return country;
+  }
+  return undefined;
+}
+
+/** The fee for one SNCF train between two stops, or undefined when no fee page covers it. */
+export function interrailFee(axe: string | undefined, origin: string, destination: string): Fee | undefined {
+  const abroad = foreignCountry(origin) ?? foreignCountry(destination);
+  if (abroad === "ES") return [origin, destination].some((s) => fold(s).includes(" paris ")) ? SPAIN_FROM_PARIS : SPAIN_OTHER;
+  if (abroad) return BY_COUNTRY[abroad];
+  return axe ? BY_AXE[axe] : undefined;
+}
+
+/** The fees of every train in a journey added up, or undefined when one train has none. */
+export function journeyFee(j: Journey): Fee | undefined {
+  let min = 0;
+  let max = 0;
+  for (const leg of j.legs) {
+    const fee = interrailFee(leg.axe, leg.origin, leg.destination);
+    if (!fee) return undefined;
+    min += fee.min;
+    max += fee.max;
+  }
+  return { min, max };
+}
+
+/** "€19" or "€10–20", in the reader's locale. */
+export function formatFee(fee: Fee, lang: string): string {
+  const eur = (v: number): string =>
+    new Intl.NumberFormat(lang, { style: "currency", currency: "EUR", maximumFractionDigits: 0 }).format(v);
+  return fee.min === fee.max ? eur(fee.min) : `${eur(fee.min)}–${eur(fee.max)}`;
+}
diff --git a/src/i18n/ar.ts b/src/i18n/ar.ts
index 7ca09be..cbfbc8d 100644
--- a/src/i18n/ar.ts
+++ b/src/i18n/ar.ts
@@ -108,6 +108,8 @@ export const ar: Dict = {
   set_show_map_hint: "أوقفها لتطبيق أخفّ وأسرع على الهواتف الضعيفة.",
   set_compact: "نتائج مضغوطة",
   set_compact_hint: "مزيد من القطارات في الشاشة.",
+  set_fees: "رسوم حجز إنتررييل",
+  set_fees_hint: "يعرض تكلفة الحجز الإلزامي لكل قطار، في الدرجة الثانية.",
   set_lowend: "جهاز ضعيف",
   set_lowend_hint: "تقليل الحركة وإخفاء الخريطة وضغط القائمة بضغطة واحدة.",
   lowend_prompt_title: "تسريع التطبيق؟",
@@ -250,6 +252,7 @@ export const ar: Dict = {
   sort_fastest: "الأسرع",
   sort_arrival: "الوصول الأبكر",
   sort_departure: "المغادرة الأبكر",
+  sort_cheapest: "أرخص حجز",
   sort_name: "أ–ي",
 
   mode_oneway: "ذهاب فقط",
@@ -302,6 +305,9 @@ export const ar: Dict = {
   how_eco: "البيئة: رحلة بقطار TGV تُصدر نحو 50 مرة أقل من ثاني أكسيد الكربون مقارنة بالسيارة لشخص واحد، و100 مرة أقل من الطائرة (أرقام ADEME).",
   eco_trip: "بالقطار، تُصدر هذه الرحلة نحو {train} كغ من مكافئ CO₂، مقابل {car} كغ بالسيارة لشخص واحد و{plane} كغ بالطائرة (ADEME، {km} كم في خط مستقيم).",
   eco_link: "أرقام CO₂ من ADEME",
+  fee_chip: "الحجز {fee}",
+  fee_note: "رسوم حجز إنتررييل تقريبية، الدرجة الثانية.",
+  fee_link: "رسوم إنتررييل",
   tagline_ir: "قطارات SNCF بتذكرة Interrail.",
   how_note_ir: "تعرض هذه الأداة القطارات التي تسير (البيانات المفتوحة لـ SNCF)؛ ولا تعرف إن بقي مقعد لحاملي التذكرة. تحقق على SNCF Connect.",
   multi_incomplete_ir: "بعض المقاطع لا يوجد فيها قطار — هذا المسار غير مكتمل.",
diff --git a/src/i18n/de.ts b/src/i18n/de.ts
index 3cd507b..b54f21e 100644
--- a/src/i18n/de.ts
+++ b/src/i18n/de.ts
@@ -108,6 +108,8 @@ export const de: Dict = {
   set_show_map_hint: "Aus für eine leichtere, schnellere App auf schwachen Handys.",
   set_compact: "Kompakte Ergebnisse",
   set_compact_hint: "Mehr Züge pro Bildschirm.",
+  set_fees: "Interrail-Reservierungsgebühren",
+  set_fees_hint: "Zeigt, was die Pflichtreservierung jedes Zuges in der 2. Klasse kostet.",
   set_lowend: "Schwaches Gerät",
   set_lowend_hint: "Bewegung reduzieren, Karte ausblenden und Liste kompakt — mit einem Tipp.",
   lowend_prompt_title: "App beschleunigen?",
@@ -251,6 +253,7 @@ export const de: Dict = {
   sort_fastest: "Schnellste",
   sort_arrival: "Früheste Ankunft",
   sort_departure: "Früheste Abfahrt",
+  sort_cheapest: "Günstigste Reservierung",
   sort_name: "A–Z",
 
   mode_oneway: "Einfache Fahrt",
@@ -303,6 +306,9 @@ export const de: Dict = {
   how_eco: "Ökologie: Eine TGV-Fahrt verursacht etwa 50-mal weniger CO₂ als allein mit dem Auto und 100-mal weniger als mit dem Flugzeug (ADEME-Zahlen).",
   eco_trip: "Mit dem Zug verursacht diese Fahrt etwa {train} kg CO₂e, gegenüber {car} kg allein im Auto und {plane} kg im Flugzeug (ADEME, {km} km Luftlinie).",
   eco_link: "CO₂-Zahlen der ADEME",
+  fee_chip: "Reservierung {fee}",
+  fee_note: "Richtwerte für Interrail-Reservierungsgebühren, 2. Klasse.",
+  fee_link: "Interrail-Gebühren",
   tagline_ir: "SNCF-Züge mit einem Interrail-Pass.",
   how_note_ir: "Dieses Tool zeigt fahrende Züge (offene SNCF-Daten); es sieht nicht, ob noch ein Pass-Platz frei ist. Prüfe auf SNCF Connect.",
   multi_incomplete_ir: "Für einige Abschnitte gibt es keinen Zug — diese Reise ist unvollständig.",
diff --git a/src/i18n/en.ts b/src/i18n/en.ts
index 526210a..227e5ef 100644
--- a/src/i18n/en.ts
+++ b/src/i18n/en.ts
@@ -112,6 +112,8 @@ export const en: Dict = {
   set_show_map_hint: "Turn off for a lighter, faster app on low-end phones.",
   set_compact: "Compact results",
   set_compact_hint: "More trains per screen.",
+  set_fees: "Interrail reservation fees",
+  set_fees_hint: "Shows what each train's compulsory reservation costs, 2nd class.",
   set_lowend: "Low-end device",
   set_lowend_hint: "Reduce motion, hide the map and compact the list, in one tap.",
   lowend_prompt_title: "Speed up the app?",
@@ -263,6 +265,7 @@ export const en: Dict = {
   sort_fastest: "Fastest",
   sort_arrival: "Earliest arrival",
   sort_departure: "Earliest departure",
+  sort_cheapest: "Cheapest reservation",
   sort_name: "A–Z",
 
   mode_oneway: "One-way",
@@ -315,6 +318,9 @@ export const en: Dict = {
   how_eco: "Ecology: a TGV trip emits about 50 times less CO₂ than driving alone and 100 times less than flying (ADEME figures).",
   eco_trip: "By train, this trip emits about {train} kg CO₂e, against {car} kg driving alone and {plane} kg flying (ADEME, {km} km as the crow flies).",
   eco_link: "ADEME CO₂ figures",
+  fee_chip: "Reservation {fee}",
+  fee_note: "Indicative Interrail reservation fees, 2nd class.",
+  fee_link: "Interrail fees",
   tagline_ir: "SNCF trains on an Interrail pass.",
   how_note_ir: "This tool shows trains that run (SNCF open data); it can't see whether a pass-holder seat is left. Check SNCF Connect.",
   multi_incomplete_ir: "Some legs have no train — this itinerary is incomplete.",
diff --git a/src/i18n/es.ts b/src/i18n/es.ts
index 5d37612..16954c9 100644
--- a/src/i18n/es.ts
+++ b/src/i18n/es.ts
@@ -108,6 +108,8 @@ export const es: Dict = {
   set_show_map_hint: "Desactívalo para una app más ligera y rápida en móviles básicos.",
   set_compact: "Resultados compactos",
   set_compact_hint: "Más trenes por pantalla.",
+  set_fees: "Suplementos de reserva Interrail",
+  set_fees_hint: "Muestra el precio de la reserva obligatoria de cada tren, en 2.ª clase.",
   set_lowend: "Dispositivo básico",
   set_lowend_hint: "Reduce animaciones, oculta el mapa y compacta la lista, de una vez.",
   lowend_prompt_title: "¿Acelerar la app?",
@@ -251,6 +253,7 @@ export const es: Dict = {
   sort_fastest: "Más rápido",
   sort_arrival: "Llegada más temprana",
   sort_departure: "Salida más temprana",
+  sort_cheapest: "Reserva más barata",
   sort_name: "A–Z",
 
   mode_oneway: "Solo ida",
@@ -303,6 +306,9 @@ export const es: Dict = {
   how_eco: "Ecología: un viaje en TGV emite unas 50 veces menos CO₂ que en coche solo y 100 veces menos que en avión (cifras de ADEME).",
   eco_trip: "En tren, este viaje emite unos {train} kg de CO₂e, frente a {car} kg en coche solo y {plane} kg en avión (ADEME, {km} km en línea recta).",
   eco_link: "Cifras de CO₂ de ADEME",
+  fee_chip: "Reserva {fee}",
+  fee_note: "Suplementos de reserva Interrail orientativos, en 2.ª clase.",
+  fee_link: "Tarifas Interrail",
   tagline_ir: "Trenes SNCF con un pase Interrail.",
   how_note_ir: "Esta herramienta muestra los trenes que circulan (datos abiertos de SNCF); no sabe si queda plaza de pase. Verifica en SNCF Connect.",
   multi_incomplete_ir: "Algunos trayectos no tienen tren — este itinerario está incompleto.",
diff --git a/src/i18n/fr.ts b/src/i18n/fr.ts
index 1b85ca8..021e793 100644
--- a/src/i18n/fr.ts
+++ b/src/i18n/fr.ts
@@ -110,6 +110,8 @@ export const fr = {
   set_show_map_hint: "Désactivez-la pour une appli plus légère et rapide sur mobile bas de gamme.",
   set_compact: "Résultats compacts",
   set_compact_hint: "Plus de trains par écran.",
+  set_fees: "Frais de réservation Interrail",
+  set_fees_hint: "Affiche le prix de la réservation obligatoire de chaque train, en 2de classe.",
   set_lowend: "Appareil bas de gamme",
   set_lowend_hint: "Réduit les animations, masque la carte et compacte la liste, en un geste.",
   lowend_prompt_title: "Accélérer l'appli ?",
@@ -261,6 +263,7 @@ export const fr = {
   sort_fastest: "Plus rapide",
   sort_arrival: "Arrivée la plus tôt",
   sort_departure: "Départ le plus tôt",
+  sort_cheapest: "Réservation la moins chère",
   sort_name: "A–Z",
 
   mode_oneway: "Aller simple",
@@ -319,6 +322,9 @@ export const fr = {
   how_eco: "Écologie : en TGV, un trajet émet environ 50 fois moins de CO₂ qu'en voiture seul et 100 fois moins qu'en avion (chiffres ADEME).",
   eco_trip: "En train, ce trajet émet environ {train} kg de CO₂e, contre {car} kg en voiture seul et {plane} kg en avion (ADEME, {km} km à vol d'oiseau).",
   eco_link: "Calculs CO₂ de l'ADEME",
+  fee_chip: "Résa {fee}",
+  fee_note: "Frais de réservation Interrail indicatifs, en 2de classe.",
+  fee_link: "Tarifs Interrail",
   tagline_ir: "Trains SNCF avec un pass Interrail.",
   how_note_ir: "Cet outil affiche les trains qui circulent (open data SNCF) ; il ne sait pas s'il reste une place pass. Vérifiez sur SNCF Connect.",
   multi_incomplete_ir: "Certains trajets n'ont aucun train — cet itinéraire est incomplet.",
diff --git a/src/i18n/it.ts b/src/i18n/it.ts
index c8bc72e..5f1270e 100644
--- a/src/i18n/it.ts
+++ b/src/i18n/it.ts
@@ -108,6 +108,8 @@ export const it: Dict = {
   set_show_map_hint: "Disattivala per un'app più leggera e veloce su telefoni economici.",
   set_compact: "Risultati compatti",
   set_compact_hint: "Più treni per schermata.",
+  set_fees: "Supplementi di prenotazione Interrail",
+  set_fees_hint: "Mostra quanto costa la prenotazione obbligatoria di ogni treno, in 2ª classe.",
   set_lowend: "Dispositivo economico",
   set_lowend_hint: "Riduce animazioni, nasconde la mappa e compatta la lista, con un tocco.",
   lowend_prompt_title: "Velocizzare l'app?",
@@ -251,6 +253,7 @@ export const it: Dict = {
   sort_fastest: "Più veloce",
   sort_arrival: "Arrivo più presto",
   sort_departure: "Partenza più presto",
+  sort_cheapest: "Prenotazione più economica",
   sort_name: "A–Z",
 
   mode_oneway: "Solo andata",
@@ -303,6 +306,9 @@ export const it: Dict = {
   how_eco: "Ecologia: un viaggio in TGV emette circa 50 volte meno CO₂ che in auto da soli e 100 volte meno che in aereo (dati ADEME).",
   eco_trip: "In treno, questo viaggio emette circa {train} kg di CO₂e, contro {car} kg in auto da soli e {plane} kg in aereo (ADEME, {km} km in linea d'aria).",
   eco_link: "Dati CO₂ dell'ADEME",
+  fee_chip: "Prenotazione {fee}",
+  fee_note: "Supplementi di prenotazione Interrail indicativi, 2ª classe.",
+  fee_link: "Tariffe Interrail",
   tagline_ir: "Treni SNCF con un pass Interrail.",
   how_note_ir: "Questo strumento mostra i treni che circolano (dati aperti SNCF); non sa se resta un posto pass. Verifica su SNCF Connect.",
   multi_incomplete_ir: "Alcune tratte non hanno treni — questo itinerario è incompleto.",
diff --git a/src/i18n/ja.ts b/src/i18n/ja.ts
index 6ec1d4a..f856d09 100644
--- a/src/i18n/ja.ts
+++ b/src/i18n/ja.ts
@@ -108,6 +108,8 @@ export const ja: Dict = {
   set_show_map_hint: "低スペック端末では、オフにして軽く速く。",
   set_compact: "コンパクト表示",
   set_compact_hint: "1画面により多くの列車。",
+  set_fees: "インターレイル予約料金",
+  set_fees_hint: "各列車の必須予約にかかる料金を2等車で表示します。",
   set_lowend: "低スペック端末",
   set_lowend_hint: "モーション削減・地図非表示・リスト圧縮をまとめて。",
   lowend_prompt_title: "アプリを軽くしますか？",
@@ -250,6 +252,7 @@ export const ja: Dict = {
   sort_fastest: "速い順",
   sort_arrival: "到着が早い順",
   sort_departure: "出発が早い順",
+  sort_cheapest: "予約料金が安い順",
   sort_name: "A–Z",
 
   mode_oneway: "片道",
@@ -302,6 +305,9 @@ export const ja: Dict = {
   how_eco: "環境：TGVでの移動のCO₂排出は、1人で車に乗る場合の約50分の1、飛行機の約100分の1です（ADEMEの数値）。",
   eco_trip: "列車なら、この旅のCO₂e排出は約{train} kgです。1人で車なら{car} kg、飛行機なら{plane} kgです（ADEME、直線距離{km} km）。",
   eco_link: "ADEMEのCO₂データ",
+  fee_chip: "予約 {fee}",
+  fee_note: "インターレイルの予約料金（2等車）は目安です。",
+  fee_link: "インターレイル料金",
   tagline_ir: "Interrail パスで乗る SNCF の列車。",
   how_note_ir: "このツールは運行中の列車（SNCF オープンデータ）を表示します。パス席が残っているかは分かりません。SNCF Connect でご確認ください。",
   multi_incomplete_ir: "一部の区間に列車がありません — この行程は不完全です。",
diff --git a/src/i18n/ko.ts b/src/i18n/ko.ts
index 3b43ac4..7ead45a 100644
--- a/src/i18n/ko.ts
+++ b/src/i18n/ko.ts
@@ -108,6 +108,8 @@ export const ko: Dict = {
   set_show_map_hint: "저사양 폰에서 더 가볍고 빠르게 하려면 끄세요.",
   set_compact: "좁게 보기",
   set_compact_hint: "화면당 더 많은 열차.",
+  set_fees: "인터레일 예약 요금",
+  set_fees_hint: "각 열차의 필수 예약 요금을 2등석 기준으로 보여 줍니다.",
   set_lowend: "저사양 기기",
   set_lowend_hint: "모션 줄이기, 지도 숨김, 목록 압축을 한 번에.",
   lowend_prompt_title: "앱을 더 빠르게 할까요?",
@@ -251,6 +253,7 @@ export const ko: Dict = {
   sort_fastest: "빠른 순",
   sort_arrival: "가장 빠른 도착",
   sort_departure: "가장 빠른 출발",
+  sort_cheapest: "예약 요금 낮은 순",
   sort_name: "가나다순",
 
   mode_oneway: "편도",
@@ -303,6 +306,9 @@ export const ko: Dict = {
   how_eco: "환경: TGV 여행은 혼자 자동차로 갈 때보다 CO₂를 약 50배, 비행기보다 약 100배 적게 배출합니다(ADEME 수치).",
   eco_trip: "기차로 이 여정은 약 {train} kg CO₂e를 배출합니다. 혼자 자동차로는 {car} kg, 비행기로는 {plane} kg입니다(ADEME, 직선거리 {km} km).",
   eco_link: "ADEME CO₂ 수치",
+  fee_chip: "예약 {fee}",
+  fee_note: "인터레일 예약 요금(2등석)은 참고용입니다.",
+  fee_link: "인터레일 요금",
   tagline_ir: "Interrail 패스로 타는 SNCF 열차.",
   how_note_ir: "이 도구는 운행하는 열차(SNCF 공개 데이터)를 표시하며, 패스 좌석이 남았는지는 알 수 없습니다. SNCF Connect에서 확인하세요.",
   multi_incomplete_ir: "일부 구간에 열차가 없습니다 — 이 일정은 미완성입니다.",
diff --git a/src/i18n/nl.ts b/src/i18n/nl.ts
index 7a55135..7db40bf 100644
--- a/src/i18n/nl.ts
+++ b/src/i18n/nl.ts
@@ -108,6 +108,8 @@ export const nl: Dict = {
   set_show_map_hint: "Uit voor een lichtere, snellere app op zwakke telefoons.",
   set_compact: "Compacte resultaten",
   set_compact_hint: "Meer treinen per scherm.",
+  set_fees: "Interrail-reserveringskosten",
+  set_fees_hint: "Toont wat de verplichte reservering van elke trein kost, 2e klas.",
   set_lowend: "Zwak apparaat",
   set_lowend_hint: "Beweging beperken, kaart verbergen en lijst compact — in één tik.",
   lowend_prompt_title: "App sneller maken?",
@@ -250,6 +252,7 @@ export const nl: Dict = {
   sort_fastest: "Snelste",
   sort_arrival: "Vroegste aankomst",
   sort_departure: "Vroegste vertrek",
+  sort_cheapest: "Goedkoopste reservering",
   sort_name: "A–Z",
 
   mode_oneway: "Enkele reis",
@@ -302,6 +305,9 @@ export const nl: Dict = {
   how_eco: "Ecologie: een TGV-reis stoot ongeveer 50 keer minder CO₂ uit dan alleen met de auto en 100 keer minder dan vliegen (cijfers van ADEME).",
   eco_trip: "Met de trein stoot deze reis ongeveer {train} kg CO₂e uit, tegen {car} kg alleen met de auto en {plane} kg met het vliegtuig (ADEME, {km} km hemelsbreed).",
   eco_link: "CO₂-cijfers van ADEME",
+  fee_chip: "Reservering {fee}",
+  fee_note: "Indicatieve Interrail-reserveringskosten, 2e klas.",
+  fee_link: "Interrail-tarieven",
   tagline_ir: "SNCF-treinen met een Interrail-pas.",
   how_note_ir: "Deze tool toont treinen die rijden (open data SNCF); hij ziet niet of er nog een passplaats vrij is. Controleer op SNCF Connect.",
   multi_incomplete_ir: "Sommige trajecten hebben geen trein — deze reis is onvolledig.",
diff --git a/src/i18n/pt.ts b/src/i18n/pt.ts
index 4111bcd..e0c15f2 100644
--- a/src/i18n/pt.ts
+++ b/src/i18n/pt.ts
@@ -108,6 +108,8 @@ export const pt: Dict = {
   set_show_map_hint: "Desative para uma app mais leve e rápida em telemóveis modestos.",
   set_compact: "Resultados compactos",
   set_compact_hint: "Mais comboios por ecrã.",
+  set_fees: "Taxas de reserva Interrail",
+  set_fees_hint: "Mostra quanto custa a reserva obrigatória de cada comboio, em 2.ª classe.",
   set_lowend: "Dispositivo modesto",
   set_lowend_hint: "Reduz o movimento, oculta o mapa e compacta a lista, num toque.",
   lowend_prompt_title: "Acelerar o app?",
@@ -250,6 +252,7 @@ export const pt: Dict = {
   sort_fastest: "Mais rápido",
   sort_arrival: "Chegada mais cedo",
   sort_departure: "Partida mais cedo",
+  sort_cheapest: "Reserva mais barata",
   sort_name: "A–Z",
 
   mode_oneway: "Só ida",
@@ -302,6 +305,9 @@ export const pt: Dict = {
   how_eco: "Ecologia: uma viagem de TGV emite cerca de 50 vezes menos CO₂ do que de carro sozinho e 100 vezes menos do que de avião (dados da ADEME).",
   eco_trip: "De comboio, esta viagem emite cerca de {train} kg de CO₂e, contra {car} kg de carro sozinho e {plane} kg de avião (ADEME, {km} km em linha reta).",
   eco_link: "Dados de CO₂ da ADEME",
+  fee_chip: "Reserva {fee}",
+  fee_note: "Taxas de reserva Interrail indicativas, 2.ª classe.",
+  fee_link: "Taxas Interrail",
   tagline_ir: "Comboios SNCF com um passe Interrail.",
   how_note_ir: "Esta ferramenta mostra os comboios que circulam (dados abertos da SNCF); não sabe se ainda há lugar de passe. Verifique no SNCF Connect.",
   multi_incomplete_ir: "Alguns trajetos não têm comboio — este itinerário está incompleto.",
diff --git a/src/i18n/zh.ts b/src/i18n/zh.ts
index d9500ac..5097400 100644
--- a/src/i18n/zh.ts
+++ b/src/i18n/zh.ts
@@ -108,6 +108,8 @@ export const zh: Dict = {
   set_show_map_hint: "在低端手机上关闭可让应用更轻更快。",
   set_compact: "紧凑结果",
   set_compact_hint: "每屏显示更多班次。",
+  set_fees: "Interrail 订座费",
+  set_fees_hint: "显示每趟列车必须订座的费用（二等座）。",
   set_lowend: "低端设备",
   set_lowend_hint: "一键减少动效、隐藏地图并压缩列表。",
   lowend_prompt_title: "让应用更快？",
@@ -250,6 +252,7 @@ export const zh: Dict = {
   sort_fastest: "最快",
   sort_arrival: "最早到达",
   sort_departure: "最早出发",
+  sort_cheapest: "订座费最低",
   sort_name: "A–Z",
 
   mode_oneway: "单程",
@@ -302,6 +305,9 @@ export const zh: Dict = {
   how_eco: "环保：乘坐 TGV 出行的二氧化碳排放约为独自驾车的五十分之一、乘飞机的百分之一（ADEME 数据）。",
   eco_trip: "乘火车，这段行程约排放 {train} kg CO₂e，独自驾车为 {car} kg，乘飞机为 {plane} kg（ADEME，直线距离 {km} km）。",
   eco_link: "ADEME 二氧化碳数据",
+  fee_chip: "订座 {fee}",
+  fee_note: "Interrail 订座费仅供参考（二等座）。",
+  fee_link: "Interrail 费用",
   tagline_ir: "持 Interrail 通票乘坐的 SNCF 列车。",
   how_note_ir: "本工具显示运行中的列车（SNCF 开放数据），无法得知是否还有通票座位。请在 SNCF Connect 上核实。",
   multi_incomplete_ir: "部分行程没有列车——此行程不完整。",
diff --git a/src/state/store.ts b/src/state/store.ts
index 58ffe83..4e707bd 100644
--- a/src/state/store.ts
+++ b/src/state/store.ts
@@ -40,6 +40,8 @@ export interface Settings {
   /** Show the interactive map. Off = no Leaflet/tiles at all: the biggest saving on
    *  low-end devices and slow connections; results go full-width. */
   map: boolean;
+  /** Show Interrail pass-holder reservation fees on each train (off by default). */
+  fees: boolean;
 }
 
 export type Density = "comfortable" | "compact";
@@ -87,6 +89,7 @@ export function loadSettings(): Settings {
     density: s.density === "compact" ? "compact" : "comfortable",
     reduceMotion: s.reduceMotion === true,
     map: s.map !== false, // default on
+    fees: s.fees === true,
   };
 }
 
@@ -307,7 +310,7 @@ export function queryToParams(q: SearchQuery): URLSearchParams {
   return p;
 }
 
-const SORT_KEYS = ["rec", "trains", "days", "closest", "fastest", "name"] as const;
+const SORT_KEYS = ["rec", "trains", "days", "closest", "fastest", "name", "cheapest"] as const;
 
 export function queryFromParams(p: URLSearchParams, fallbackDate: string): SearchQuery {
   const rawMode = p.get("mode") ?? "from";
diff --git a/src/styles.css b/src/styles.css
index 2c04e49..e19b933 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -1992,6 +1992,17 @@ a {
   color: var(--muted);
   font-size: 0.83rem;
 }
+/* The route chip and, when turned on, the Interrail reservation fee beside it. */
+.journey-tags {
+  display: inline-flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.35rem;
+  min-width: 0;
+}
+.chip-fee {
+  color: var(--text);
+}
 .legs {
   display: flex;
   flex-direction: column;
diff --git a/src/types.ts b/src/types.ts
index 4e5e679..9a81abb 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -163,7 +163,7 @@ export interface SearchQuery {
 }
 
 /** Result ordering for the destination / ideas lists. */
-export type SortKey = "rec" | "trains" | "days" | "closest" | "fastest" | "name" | "arrival" | "departure";
+export type SortKey = "rec" | "trains" | "days" | "closest" | "fastest" | "name" | "arrival" | "departure" | "cheapest";
 
 /** A direct or multi-leg (1..3 legs) journey. */
 export interface Journey {
diff --git a/src/ui/modals.ts b/src/ui/modals.ts
index d082c20..1d2b852 100644
--- a/src/ui/modals.ts
+++ b/src/ui/modals.ts
@@ -134,9 +134,12 @@ export function showSettingsModal(opts: {
   reduceMotion: boolean;
   map: boolean;
   compact: boolean;
+  /** Interrail reservation fees on each train (off by default). */
+  fees: boolean;
   onReduceMotion: (v: boolean) => void;
   onMap: (v: boolean) => void;
   onCompact: (v: boolean) => void;
+  onFees: (v: boolean) => void;
   /** Master "Low-end mode": ON = all three savers on, OFF = all three off. */
   onLowEnd: (v: boolean) => void;
 }): void {
@@ -175,6 +178,12 @@ export function showSettingsModal(opts: {
       el("p", { class: "modal-text muted small", text: t("set_perf") }),
       el("div", { class: "set-rows set-master" }, [master]),
       body,
+      el("div", { class: "set-rows" }, [
+        settingSwitch(t("set_fees"), t("set_fees_hint"), opts.fees, (v) => {
+          opts.onFees(v);
+          rebuild({ fees: v });
+        }),
+      ]),
       el("p", {
         class: "muted small set-version",
         text: `Railmap v${APP_VERSION}${APP_BUILD ? ` · ${APP_BUILD}` : ""}`,
diff --git a/src/ui/render.ts b/src/ui/render.ts
index 0bcb8b0..2b54468 100644
--- a/src/ui/render.ts
+++ b/src/ui/render.ts
@@ -9,7 +9,8 @@ import { el } from "./dom";
 import { isNightTrain } from "../core/search";
 import { isAirportStation } from "../data/stations";
 import { formatDuration, dayIndex, addDays } from "../util/time";
-import { t } from "../i18n";
+import { t, getLang } from "../i18n";
+import { formatFee, type Fee } from "../data/fees";
 
 // Monotonic id source for wiring aria-labelledby (a calendar's <h3> to its grid).
 let uidSeq = 0;
@@ -36,6 +37,8 @@ export interface RenderCtx {
   onIcs: (journey: Journey) => void;
   /** Open the step-by-step "book each train" modal for a connecting journey. */
   onBookSteps: (journey: Journey) => void;
+  /** A journey's Interrail reservation fees, set only when the traveller turned them on. */
+  fee?: (journey: Journey) => Fee | undefined;
   isFavorite: (route: RoutePair) => boolean;
   onToggleFavorite: (route: RoutePair) => void;
   /** Whether this trip (one-way, or a round trip with `inbound`) is saved. */
@@ -592,12 +595,18 @@ function journeyBodyEl(j: Journey, ctx: RenderCtx, label?: string, dateLabel?: s
           class: "chip chip-via",
           text: t("lbl_via", { hub: j.hubs.map((h) => ctx.label(h)).join(", ") }),
         });
+  const fee = ctx.fee?.(j);
   const head = el("div", { class: "journey-head" }, [
     ...(label ? [el("h3", { class: "trip-leg-title", text: label })] : []),
     // With flexible dates the list spans several days, so each card carries its own
     // date (otherwise which day a proposition is for is lost). Leads the head.
     ...(dateLabel ? [el("span", { class: "chip chip-date", text: dateLabel })] : []),
-    tag,
+    fee
+      ? el("span", { class: "journey-tags" }, [
+          tag,
+          el("span", { class: "chip chip-fee", text: t("fee_chip", { fee: formatFee(fee, getLang()) }) }),
+        ])
+      : tag,
     el("span", { class: "journey-total" }, [icon(I.clock), el("span", { text: formatDuration(j.totalDurationMin) })]),
   ]);
   return el("div", { class: "journey-body" }, [head, legs]);
diff --git a/tests/fees.test.ts b/tests/fees.test.ts
new file mode 100644
index 0000000..3c73e65
--- /dev/null
+++ b/tests/fees.test.ts
@@ -0,0 +1,42 @@
+import { describe, it, expect } from "vitest";
+import { formatFee, interrailFee, journeyFee } from "../src/data/fees";
+import type { Journey, MaxTrain } from "../src/types";
+
+const leg = (origin: string, destination: string, axe?: string): MaxTrain => ({
+  date: "2026-10-19",
+  origin,
+  destination,
+  depart: "08:00",
+  arrive: "10:00",
+  departMin: 480,
+  arriveMin: 600,
+  durationMin: 120,
+  trainNo: "6201",
+  available: true,
+  axe,
+});
+
+describe("Interrail reservation fees", () => {
+  it("prices a train by the country abroad, else by its axe", () => {
+    expect(interrailFee("SUD EST", "PARIS (intramuros)", "MARSEILLE ST CHARLES")).toEqual({ min: 10, max: 20 });
+    expect(interrailFee("INTERNATIONAL", "PARIS (intramuros)", "STUTTGART HBF")).toEqual({ min: 19, max: 19 });
+    expect(interrailFee("INTERNATIONAL", "PARIS (intramuros)", "ZURICH HB")).toEqual({ min: 25, max: 25 });
+    expect(interrailFee("INTERNATIONAL", "LYON (intramuros)", "TORINO PORTA SUSA")).toEqual({ min: 31, max: 31 });
+    expect(interrailFee("INTERNATIONAL", "PARIS (intramuros)", "DIJON VILLE")).toEqual({ min: 10, max: 20 });
+    expect(interrailFee("IC NUIT", "PARIS (intramuros)", "BRIANCON")).toEqual({ min: 10, max: 25 });
+    expect(interrailFee("AUTOCAR SNCF", "PARIS (intramuros)", "ROUEN RIVE DROITE")).toBeUndefined();
+    // Whole words only: Bernay is in Normandy, not Bern.
+    expect(interrailFee("NORD", "PARIS (intramuros)", "BERNAY")).toEqual({ min: 10, max: 20 });
+  });
+
+  it("adds up a connecting journey's fees, and gives none when one train has none", () => {
+    const j = (legs: MaxTrain[]) => ({ legs }) as unknown as Journey;
+    expect(journeyFee(j([leg("PARIS (intramuros)", "LYON (intramuros)", "SUD EST"), leg("LYON (intramuros)", "TORINO PORTA SUSA", "INTERNATIONAL")]))).toEqual({ min: 41, max: 51 });
+    expect(journeyFee(j([leg("PARIS (intramuros)", "ROUEN RIVE DROITE", "AUTOCAR SNCF")]))).toBeUndefined();
+  });
+
+  it("writes a fixed fee as one price and a range as two", () => {
+    expect(formatFee({ min: 19, max: 19 }, "en")).toBe("€19");
+    expect(formatFee({ min: 10, max: 20 }, "en")).toBe("€10–€20");
+  });
+});