--- a/ai/trolly/build.c
+++ b/ai/trolly/build.c
@@ -220,28 +220,30 @@
 
 
 // This functions tries to find the best vehicle for this type of cargo
-// It returns vehicle_id or -1 if not found
-int AiNew_PickVehicle(Player *p)
+// It returns INVALID_ENGINE if not suitable engine is found
+EngineID AiNew_PickVehicle(Player *p)
 {
 	if (p->ainew.tbt == AI_TRAIN) {
 		// Not supported yet
-		return -1;
+		return INVALID_ENGINE;
 	} else {
-		int start, count, i, ret = CMD_ERROR;
-		start = _cargoc.ai_roadveh_start[p->ainew.cargo];
-		count = _cargoc.ai_roadveh_count[p->ainew.cargo];
+		EngineID start = _cargoc.ai_roadveh_start[p->ainew.cargo];
+		EngineID end = start + _cargoc.ai_roadveh_count[p->ainew.cargo];
+		EngineID i;
 
 		// Let's check it backwards.. we simply want to best engine available..
-		for (i = start + count - 1; i >= start; i--) {
+		for (i = end - 1; i >= start; i--) {
+			int32 ret;
+
 			// Is it availiable?
 			// Also, check if the reliability of the vehicle is above the AI_VEHICLE_MIN_RELIABILTY
 			if (!HASBIT(GetEngine(i)->player_avail, _current_player) || GetEngine(i)->reliability * 100 < AI_VEHICLE_MIN_RELIABILTY << 16) continue;
 			// Can we build it?
 			ret = AI_DoCommand(0, i, 0, DC_QUERY_COST, CMD_BUILD_ROAD_VEH);
-			if (!CmdFailed(ret)) break;
+			if (!CmdFailed(ret)) return i;
 		}
 		// We did not find a vehicle :(
-		return CmdFailed(ret) ? -1 : i;
+		return INVALID_ENGINE;
 	}
 }
 
@@ -249,9 +251,9 @@
 // Builds the best vehicle possible
 int AiNew_Build_Vehicle(Player *p, TileIndex tile, byte flag)
 {
-	int i = AiNew_PickVehicle(p);
-	if (i == -1) return CMD_ERROR;
+	EngineID i = AiNew_PickVehicle(p);
 
+	if (i == INVALID_ENGINE) return CMD_ERROR;
 	if (p->ainew.tbt == AI_TRAIN) return CMD_ERROR;
 
 	return AI_DoCommand(tile, i, 0, flag, CMD_BUILD_ROAD_VEH);

--- a/ai/trolly/trolly.c
+++ b/ai/trolly/trolly.c
@@ -567,7 +567,8 @@
 {
 	TileIndex tile;
 	Station *st;
-	int i, count = 0;
+	int count = 0;
+	EngineID i;
 	TileIndex new_tile = 0;
 	byte direction = 0;
 	Town *town = NULL;
@@ -606,7 +607,10 @@
 	i = AiNew_PickVehicle(p);
 	// Euhmz, this should not happen _EVER_
 	// Quit finding a route...
-	if (i == -1) { p->ainew.state = AI_STATE_NOTHING; return; }
+	if (i == INVALID_ENGINE) {
+		p->ainew.state = AI_STATE_NOTHING;
+		return;
+	}
 
 	FOR_ALL_STATIONS(st) {
 		if (st->xy != 0) {
@@ -867,10 +871,11 @@
 {
 	if (p->ainew.tbt == AI_BUS) {
 		// For bus-routes we look at the time before we are back in the station
-		int i, length, tiles_a_day;
+		EngineID i;
+		int length, tiles_a_day;
 		int amount;
 		i = AiNew_PickVehicle(p);
-		if (i == -1) return 0;
+		if (i == INVALID_ENGINE) return 0;
 		// Passenger run.. how long is the route?
 		length = p->ainew.path_info.route_length;
 		// Calculating tiles a day a vehicle moves is not easy.. this is how it must be done!
@@ -882,10 +887,11 @@
 		return amount;
 	} else if (p->ainew.tbt == AI_TRUCK) {
 		// For truck-routes we look at the cargo
-		int i, length, amount, tiles_a_day;
+		EngineID i;
+		int length, amount, tiles_a_day;
 		int max_cargo;
 		i = AiNew_PickVehicle(p);
-		if (i == -1) return 0;
+		if (i == INVALID_ENGINE) return 0;
 		// Passenger run.. how long is the route?
 		length = p->ainew.path_info.route_length;
 		// Calculating tiles a day a vehicle moves is not easy.. this is how it must be done!
@@ -1122,7 +1128,7 @@
 
 	p->ainew.state = AI_STATE_BUILD_VEHICLE;
 	p->ainew.idle = 10;
-	p->ainew.veh_main_id = (VehicleID)-1;
+	p->ainew.veh_main_id = INVALID_VEHICLE;
 }
 
 
@@ -1181,7 +1187,7 @@
 		p->ainew.veh_id = _new_roadveh_id;
 	}
 
-	if (p->ainew.veh_main_id != (VehicleID)-1) {
+	if (p->ainew.veh_main_id != INVALID_VEHICLE) {
 		AI_DoCommand(0, p->ainew.veh_id + (p->ainew.veh_main_id << 16), 0, DC_EXEC, CMD_CLONE_ORDER);
 
 		p->ainew.state = AI_STATE_START_VEHICLE;

--- a/ai/trolly/trolly.h
+++ b/ai/trolly/trolly.h
@@ -254,7 +254,7 @@
 int AiNew_Build_Station(Player *p, byte type, TileIndex tile, byte length, byte numtracks, byte direction, byte flag);
 int AiNew_Build_Bridge(Player *p, TileIndex tile_a, TileIndex tile_b, byte flag);
 int AiNew_Build_RoutePart(Player *p, Ai_PathFinderInfo *PathFinderInfo, byte flag);
-int AiNew_PickVehicle(Player *p);
+EngineID AiNew_PickVehicle(Player *p);
 int AiNew_Build_Vehicle(Player *p, TileIndex tile, byte flag);
 int AiNew_Build_Depot(Player* p, TileIndex tile, DiagDirection direction, byte flag);