--- a/aircraft_cmd.c	Thu Jan 27 20:38:19 2005 +0000
+++ b/aircraft_cmd.c	Thu Jan 27 21:00:05 2005 +0000
@@ -161,6 +161,8 @@
 	const AircraftVehicleInfo *avi = AircraftVehInfo(p1);
 	Engine *e;
 
+	if (!(IsEngineBuildable(p1, VEH_Aircraft))) return CMD_ERROR;
+
 	SET_EXPENSES_TYPE(EXPENSES_NEW_VEHICLES);
 
 	value = EstimateAircraftCost(p1);

--- a/engine.c	Thu Jan 27 20:38:19 2005 +0000
+++ b/engine.c	Thu Jan 27 21:00:05 2005 +0000
@@ -906,5 +906,29 @@
 	{ 'ENGS', LoadSave_ENGS, LoadSave_ENGS, CH_RIFF | CH_LAST},
 };
 
+/*
+ * returns true if an engine is valid, and it is of the specified type, and buildable by the current player, false otherwise
+ *
+ * engine = index of the engine to check
+ * type   = the type the engine should be of (VEH_xxx)
+ */
+bool IsEngineBuildable(int engine, byte type) {
+	Engine *e;
+
+	// check if it's an engine that is in the engine array
+	if (0 > engine || engine >= TOTAL_NUM_ENGINES ) return false;
+
+	e = DEREF_ENGINE(engine);
+
+	// check if it's an engine of specified type
+	if (e->type != type) return false;
+
+	// check if it's available
+	if (!HASBIT(e->player_avail, _current_player)) return false;
+
+	return true;
+}
 
 
+
+

--- a/engine.h	Thu Jan 27 20:38:19 2005 +0000
+++ b/engine.h	Thu Jan 27 21:00:05 2005 +0000
@@ -133,6 +133,7 @@
 void LoadCustomEngineNames(void);
 void DeleteCustomEngineNames(void);
 
+bool IsEngineBuildable(int engine, byte type);
 
 enum {
 	NUM_NORMAL_RAIL_ENGINES = 54,

--- a/roadveh_cmd.c	Thu Jan 27 20:38:19 2005 +0000
+++ b/roadveh_cmd.c	Thu Jan 27 21:00:05 2005 +0000
@@ -115,6 +115,8 @@
 	uint tile = TILE_FROM_XY(x,y);
 	Engine *e;
 
+	if (!(IsEngineBuildable(p1, VEH_Road))) return CMD_ERROR;
+
 	SET_EXPENSES_TYPE(EXPENSES_NEW_VEHICLES);
 
 	cost = EstimateRoadVehCost(p1);

--- a/ship_cmd.c	Thu Jan 27 20:38:19 2005 +0000
+++ b/ship_cmd.c	Thu Jan 27 21:00:05 2005 +0000
@@ -816,6 +816,8 @@
 	uint tile = TILE_FROM_XY(x,y);
 	Engine *e;
 
+	if (!(IsEngineBuildable(p1, VEH_Ship))) return CMD_ERROR;
+
 	SET_EXPENSES_TYPE(EXPENSES_NEW_VEHICLES);
 
 	value = EstimateShipCost(p1);

--- a/train_cmd.c	Thu Jan 27 20:38:19 2005 +0000
+++ b/train_cmd.c	Thu Jan 27 21:00:05 2005 +0000
@@ -525,6 +525,8 @@
 	Engine *e;
 	uint tile;
 
+	if (!(IsEngineBuildable(p1, VEH_Train))) return CMD_ERROR;
+
 	_cmd_build_rail_veh_var1 = 0;
 
 	SET_EXPENSES_TYPE(EXPENSES_NEW_VEHICLES);