Allow converting track under trains when compatible with the new rail type

src/rail_cmd.cpp

@@ -2027,6 +2027,70 @@ static Vehicle *UpdateTrainPowerProc(Vehicle *v, void *data)
 	return nullptr;
 }
 
+struct EnsureNoIncompatibleRailtypeTrainOnGroundData {
+	int z;
+	RailType type;
+};
+
+static Vehicle *EnsureNoIncompatibleRailtypeTrainProc(Vehicle *v, void *data)
+{
+	const EnsureNoIncompatibleRailtypeTrainOnGroundData *procdata = (EnsureNoIncompatibleRailtypeTrainOnGroundData *)data;
+
+	if (v->z_pos > procdata->z) return nullptr;
+	if (HasBit(Train::From(v)->First()->compatible_railtypes, procdata->type)) return nullptr;
+
+	return v;
+}
+
+CommandCost EnsureNoIncompatibleRailtypeTrainOnGround(TileIndex tile, RailType type)
+{
+	EnsureNoIncompatibleRailtypeTrainOnGroundData data = {
+		GetTileMaxPixelZ(tile),
+		type
+	};
+
+	if (HasVehicleOnPos(tile, VEH_TRAIN, &data, &EnsureNoIncompatibleRailtypeTrainProc)) {
+		return_cmd_error(STR_ERROR_TRAIN_IN_THE_WAY);
+	}
+	return CommandCost();
+}
+
+struct EnsureNoIncompatibleRailtypeTrainOnTrackBitsData {
+	TrackBits track_bits;
+	RailType type;
+};
+
+static Vehicle *EnsureNoIncompatibleRailtypeTrainOnTrackProc(Vehicle *v, void *data)
+{
+	const EnsureNoIncompatibleRailtypeTrainOnTrackBitsData *procdata = (EnsureNoIncompatibleRailtypeTrainOnTrackBitsData *)data;
+	TrackBits rail_bits = procdata->track_bits;
+
+	Train *t = Train::From(v);
+	if (HasBit(t->First()->compatible_railtypes, procdata->type)) return nullptr;
+	if (rail_bits & TRACK_BIT_WORMHOLE) {
+		if (t->track & TRACK_BIT_WORMHOLE) return v;
+		rail_bits &= ~TRACK_BIT_WORMHOLE;
+	} else if (t->track & TRACK_BIT_WORMHOLE) {
+		return nullptr;
+	}
+	if ((t->track != rail_bits) && !TracksOverlap(t->track | rail_bits)) return nullptr;
+
+	return v;
+}
+
+CommandCost EnsureNoIncompatibleRailtypeTrainOnTrackBits(TileIndex tile, TrackBits track_bits, RailType type)
+{
+	EnsureNoIncompatibleRailtypeTrainOnTrackBitsData data = {
+		track_bits,
+		type
+	};
+
+	if (HasVehicleOnPos(tile, VEH_TRAIN, &data, &EnsureNoIncompatibleRailtypeTrainOnTrackProc)) {
+		return_cmd_error(STR_ERROR_TRAIN_IN_THE_WAY);
+	}
+	return CommandCost();
+}
+
 /**
  * Convert one rail type to the other. You can convert normal rail to
  * monorail/maglev easily or vice-versa.
@@ -2119,7 +2183,7 @@ CommandCost CmdConvertRail(TileIndex tile, DoCommandFlag flags, uint32 p1, uint3
 		 * Tunnels and bridges have special check later */
 		if (tt != MP_TUNNELBRIDGE) {
 			if (!IsCompatibleRail(type, totype) || !IsCompatibleRail(secondary_type, totype)) {
-				CommandCost ret = IsPlainRailTile(tile) ? EnsureNoTrainOnTrackBits(tile, GetTrackBits(tile)) : EnsureNoVehicleOnGround(tile);
+				CommandCost ret = IsPlainRailTile(tile) ? EnsureNoIncompatibleRailtypeTrainOnTrackBits(tile, GetTrackBits(tile), totype) : EnsureNoIncompatibleRailtypeTrainOnGround(tile, totype);
 				if (ret.Failed()) {
 					error = ret;
 					continue;