reduce train gold after each city (#3400)

## Description:

Now that cities snap to existing rails, it's possible to line up dozens
of cities in a row, producing way too much gold. This PR reduces the
gold after each stop to prevent that. Gold only starts decreasing after
the 3rd city.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/core/game/TrainStation.ts

@@ -20,7 +20,12 @@ class TradeStationStopHandler implements TrainStopHandler {
   ): void {
     const stationOwner = station.unit.owner();
     const trainOwner = trainExecution.owner();
-    const gold = mg.config().trainGold(rel(trainOwner, stationOwner));
+    const gold = mg
+      .config()
+      .trainGold(
+        rel(trainOwner, stationOwner),
+        trainExecution.tradeStopsVisited(),
+      );
     // Share revenue with the station owner if it's not the current player
     if (trainOwner !== stationOwner) {
       stationOwner.addGold(gold, station.tile());