Add dynamic railroad fares based on segment length and congestion

- Track train counts per railroad segment
- expose a fare API on segments
- charge players when entering each segment with visual feedback

src/core/execution/TrainExecution.ts

@@ -1,6 +1,7 @@
 import {
   Execution,
   Game,
+  MessageType,
   Player,
   TrainType,
   Unit,
@@ -107,6 +108,29 @@ export class TrainExecution implements Execution {
     };
   }
 
+  private enterRailroad(railroad: OrientedRailroad) {
+    const rail = railroad.getRailroad();
+    rail.incrementTrainCount();
+    const fare = rail.getFare();
+    this.player.addGold(-fare, railroad.getStart().tile());
+    if (this.mg) {
+      this.mg.displayMessage(
+        "Paid railroad fare",
+        MessageType.RECEIVED_GOLD_FROM_TRADE,
+        this.player.id(),
+        -fare,
+      );
+    }
+  }
+
+  private leaveRailroad() {
+    if (!this.currentRailroad) {
+      return;
+    }
+    const rail = this.currentRailroad.getRailroad();
+    rail.decrementTrainCount();
+  }
+
   init(mg: Game, ticks: number): void {
     this.mg = mg;
 
@@ -215,6 +239,8 @@ export class TrainExecution implements Execution {
 
   private deleteTrain() {
     this.active = false;
+    this.leaveRailroad();
+
     if (this.train?.isActive()) {
       this.train.delete(false);
     }
@@ -337,8 +363,11 @@ export class TrainExecution implements Execution {
       // This should happen after arrival processing but before departure
       this.journeyHopCount++;
 
+      // Move to the next station and railroad, updating fare/usage tracking
       this.currentStation = nextHop;
+      this.leaveRailroad();
       this.currentRailroad = railroad;
+      this.enterRailroad(railroad);
       this.currentTile = 0;
     }
 

src/core/game/Railroad.ts

@@ -4,6 +4,8 @@ import { GameUpdateType, RailTile, RailType } from "./GameUpdates";
 import { TrainStation } from "./TrainStation";
 
 export class Railroad {
+  private trainCount: number = 0;
+
   constructor(
     public from: TrainStation,
     public to: TrainStation,
@@ -23,6 +25,29 @@ export class Railroad {
     this.from.removeRailroad(this);
     this.to.removeRailroad(this);
   }
+
+  incrementTrainCount(): void {
+    this.trainCount++;
+  }
+
+  decrementTrainCount(): void {
+    this.trainCount = Math.max(0, this.trainCount - 1);
+  }
+
+  getLength(): number {
+    return this.tiles.length;
+  }
+
+  /**
+   * Dynamic fare based on railroad length and current busyness.
+   * Longer railroads cost more, but overcrowded edges become less profitable.
+   * Uses integer math to keep amounts precise.
+   */
+  getFare(): bigint {
+    const baseFare = BigInt(this.getLength() * 100); // Base fare proportional to length
+    const busynessFactor = Math.max(1, 10 - this.trainCount); // 10,9,...,1 (capped)
+    return (baseFare * BigInt(busynessFactor)) / 10n;
+  }
 }
 
 export function getOrientedRailroad(
@@ -54,6 +79,10 @@ export class OrientedRailroad {
     return this.tiles;
   }
 
+  getRailroad(): Railroad {
+    return this.railroad;
+  }
+
   getStart(): TrainStation {
     return this.forward ? this.railroad.from : this.railroad.to;
   }