Fix train unit ID allocation when moving engine to new train, reaching train limit

See: #725

src/train_cmd.cpp

@@ -2273,14 +2273,15 @@ CommandCost CmdMoveRailVehicle(TileIndex tile, DoCommandFlag flags, uint32_t p1,
 			SetWindowDirty(WC_COMPANY, _current_company);
 		}
 
-		/* Add new heads to statistics */
-		if (src_head != nullptr && src_head->IsFrontEngine()) GroupStatistics::CountVehicle(src_head, 1);
-		if (dst_head != nullptr && dst_head->IsFrontEngine()) GroupStatistics::CountVehicle(dst_head, 1);
-
 		/* Handle 'new engine' part of cases #1b, #2b, #3b, #4b and #5 in NormaliseTrainHead. */
 		NormaliseTrainHead(src_head);
 		NormaliseTrainHead(dst_head);
 
+		/* Add new heads to statistics.
+		 * This should be done after NormaliseTrainHead due to engine total limit checks in GetFreeUnitNumber. */
+		if (src_head != nullptr && src_head->IsFrontEngine()) GroupStatistics::CountVehicle(src_head, 1);
+		if (dst_head != nullptr && dst_head->IsFrontEngine()) GroupStatistics::CountVehicle(dst_head, 1);
+
 		if ((flags & DC_NO_CARGO_CAP_CHECK) == 0) {
 			CheckCargoCapacity(src_head);
 			CheckCargoCapacity(dst_head);