(svn r3058) The default AI may not build dual headed trains under certain circumstances.

Simplify this check by testing the rail vehicle info flags instead of passing a global variable around.
Note: This is not exactly the original behaviour, because the Lev4 was always allowed to be built, but i guess that was a glitch.

ai/default/default.c

@@ -133,12 +133,16 @@ static int AiChooseTrainToBuild(byte railtype, int32 money, byte flag, TileIndex
 		const RailVehicleInfo *rvi = RailVehInfo(i);
 		const Engine* e = GetEngine(i);
 
-		if (e->railtype != railtype || rvi->flags & RVI_WAGON
+		if (e->railtype != railtype ||
-		    || !HASBIT(e->player_avail, _current_player) || e->reliability < 0x8A3D)
+				rvi->flags & RVI_WAGON ||
+				(rvi->flags & RVI_MULTIHEAD && flag & 1) ||
+				!HASBIT(e->player_avail, _current_player) ||
+				e->reliability < 0x8A3D) {
 			continue;
+		}
 
 		ret = DoCommandByTile(tile, i, 0, 0, CMD_BUILD_RAIL_VEHICLE);
-		if (!CmdFailed(ret) && (!(_cmd_build_rail_veh_var1&1) || !(flag&1)) && ret <= money &&
+		if (!CmdFailed(ret) && ret <= money &&
 				_cmd_build_rail_veh_score >= best_veh_score) {
 			best_veh_score = _cmd_build_rail_veh_score;
 			best_veh_index = i;

train_cmd.c

@@ -498,24 +498,6 @@ static void NormalizeTrainVehInDepot(Vehicle *u)
 	}
 }
 
-static const byte _railveh_unk1[] = {
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 1, 1, 0, 0, 0,
-	0, 0, 0, 0, 1, 0, 1, 0,
-	0, 1, 1, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 1,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0,
-};
-
 static const byte _railveh_score[] = {
 	1, 4, 7, 19, 20, 30, 31, 19,
 	20, 21, 22, 10, 11, 30, 31, 32,
@@ -590,8 +572,6 @@ int32 CmdBuildRailVehicle(int x, int y, uint32 flags, uint32 p1, uint32 p2)
 		if (!IsTileOwner(tile, _current_player)) return CMD_ERROR;
 	}
 
-	_cmd_build_rail_veh_var1 = 0;
-
 	SET_EXPENSES_TYPE(EXPENSES_NEW_VEHICLES);
 
 	rvi = RailVehInfo(p1);
@@ -676,7 +656,6 @@ int32 CmdBuildRailVehicle(int x, int y, uint32 flags, uint32 p1, uint32 p2)
 			InvalidateWindow(WC_COMPANY, v->owner);
 		}
 	}
-	_cmd_build_rail_veh_var1 = _railveh_unk1[p1];
 	_cmd_build_rail_veh_score = _railveh_score[p1];
 
 	InvalidateWindow(WC_REPLACE_VEHICLE, VEH_Train); // updates the replace Train window

vehicle.h

@@ -447,7 +447,6 @@ VARDEF VehicleID _new_ship_id;
 VARDEF VehicleID _new_roadveh_id;
 VARDEF uint16 _aircraft_refit_capacity;
 VARDEF byte _cmd_build_rail_veh_score;
-VARDEF byte _cmd_build_rail_veh_var1;
 
 // for each player, for each vehicle type, keep a list of the vehicles.
 //VARDEF Vehicle *_vehicle_arr[8][4];