--- a/aircraft_cmd.c	Wed Aug 09 21:02:06 2006 +0000
+++ b/aircraft_cmd.c	Wed Aug 09 21:11:45 2006 +0000
@@ -22,6 +22,7 @@
 #include "table/sprites.h"
 #include "newgrf_engine.h"
 #include "newgrf_callbacks.h"
+#include "newgrf_text.h"
 
 static bool AirportMove(Vehicle *v, const AirportFTAClass *Airport);
 static bool AirportSetBlocks(Vehicle *v, AirportFTA *current_pos, const AirportFTAClass *Airport);
@@ -420,6 +421,7 @@
 int32 CmdSellAircraft(TileIndex tile, uint32 flags, uint32 p1, uint32 p2)
 {
 	Vehicle *v;
+	uint16 callback;
 
 	if (!IsVehicleIndex(p1)) return CMD_ERROR;
 
@@ -430,6 +432,14 @@
 
 	SET_EXPENSES_TYPE(EXPENSES_NEW_VEHICLES);
 
+	/* Check if this aircraft can be started/stopped. The callback will fail or
+	 * return 0xFF if it can. */
+	callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
+	if (callback != CALLBACK_FAILED && callback != 0xFF) {
+		StringID error = GetGRFStringID(GetEngineGRFID(v->engine_type), 0xD000 + callback);
+		return_cmd_error(error);
+	}
+
 	if (flags & DC_EXEC) {
 		// Invalidate depot
 		InvalidateWindow(WC_VEHICLE_DEPOT, v->tile);

--- a/newgrf_callbacks.h	Wed Aug 09 21:02:06 2006 +0000
+++ b/newgrf_callbacks.h	Wed Aug 09 21:11:45 2006 +0000
@@ -41,6 +41,10 @@
 
 	/* Called when building a station to customize the tile layout */
 	CBID_STATION_TILE_LAYOUT        = 0x24,
+
+	/* Called when the player (or AI) tries to start or stop a vehicle. Mainly
+	 * used for preventing a vehicle from leaving the depot. */
+	CBID_VEHICLE_START_STOP_CHECK   = 0x31,
 };
 
 /**

--- a/roadveh_cmd.c	Wed Aug 09 21:02:06 2006 +0000
+++ b/roadveh_cmd.c	Wed Aug 09 21:11:45 2006 +0000
@@ -25,6 +25,7 @@
 #include "vehicle_gui.h"
 #include "newgrf_callbacks.h"
 #include "newgrf_engine.h"
+#include "newgrf_text.h"
 #include "yapf/yapf.h"
 
 static const uint16 _roadveh_images[63] = {
@@ -208,6 +209,7 @@
 int32 CmdStartStopRoadVeh(TileIndex tile, uint32 flags, uint32 p1, uint32 p2)
 {
 	Vehicle *v;
+	uint16 callback;
 
 	if (!IsVehicleIndex(p1)) return CMD_ERROR;
 
@@ -215,6 +217,14 @@
 
 	if (v->type != VEH_Road || !CheckOwnership(v->owner)) return CMD_ERROR;
 
+	/* Check if this road veh can be started/stopped. The callback will fail or
+	 * return 0xFF if it can. */
+	callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
+	if (callback != CALLBACK_FAILED && callback != 0xFF) {
+		StringID error = GetGRFStringID(GetEngineGRFID(v->engine_type), 0xD000 + callback);
+		return_cmd_error(error);
+	}
+
 	if (flags & DC_EXEC) {
 		if (IsRoadVehInDepotStopped(v)) {
 			DeleteVehicleNews(p1, STR_9016_ROAD_VEHICLE_IS_WAITING);

--- a/ship_cmd.c	Wed Aug 09 21:02:06 2006 +0000
+++ b/ship_cmd.c	Wed Aug 09 21:11:45 2006 +0000
@@ -24,6 +24,7 @@
 #include "yapf/yapf.h"
 #include "debug.h"
 #include "newgrf_callbacks.h"
+#include "newgrf_text.h"
 
 static const uint16 _ship_sprites[] = {0x0E5D, 0x0E55, 0x0E65, 0x0E6D};
 static const byte _ship_sometracks[4] = {0x19, 0x16, 0x25, 0x2A};
@@ -958,6 +959,7 @@
 int32 CmdStartStopShip(TileIndex tile, uint32 flags, uint32 p1, uint32 p2)
 {
 	Vehicle *v;
+	uint16 callback;
 
 	if (!IsVehicleIndex(p1)) return CMD_ERROR;
 
@@ -965,6 +967,14 @@
 
 	if (v->type != VEH_Ship || !CheckOwnership(v->owner)) return CMD_ERROR;
 
+	/* Check if this ship can be started/stopped. The callback will fail or
+	 * return 0xFF if it can. */
+	callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
+	if (callback != CALLBACK_FAILED && callback != 0xFF) {
+		StringID error = GetGRFStringID(GetEngineGRFID(v->engine_type), 0xD000 + callback);
+		return_cmd_error(error);
+	}
+
 	if (flags & DC_EXEC) {
 		if (IsShipInDepotStopped(v)) {
 			DeleteVehicleNews(p1, STR_981C_SHIP_IS_WAITING_IN_DEPOT);

--- a/train_cmd.c	Wed Aug 09 21:02:06 2006 +0000
+++ b/train_cmd.c	Wed Aug 09 21:11:45 2006 +0000
@@ -1227,6 +1227,7 @@
 int32 CmdStartStopTrain(TileIndex tile, uint32 flags, uint32 p1, uint32 p2)
 {
 	Vehicle *v;
+	uint16 callback;
 
 	if (!IsVehicleIndex(p1)) return CMD_ERROR;
 
@@ -1234,6 +1235,14 @@
 
 	if (v->type != VEH_Train || !CheckOwnership(v->owner)) return CMD_ERROR;
 
+	/* Check if this train can be started/stopped. The callback will fail or
+	 * return 0xFF if it can. */
+	callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
+	if (callback != CALLBACK_FAILED && callback != 0xFF) {
+		StringID error = GetGRFStringID(GetEngineGRFID(v->engine_type), 0xD000 + callback);
+		return_cmd_error(error);
+	}
+
 	if (flags & DC_EXEC) {
 		if (v->vehstatus & VS_STOPPED && v->u.rail.track == 0x80) {
 			DeleteVehicleNews(p1, STR_8814_TRAIN_IS_WAITING_IN_DEPOT);