/* Don't check callback for articulated or rear dual headed parts */

			if (!IsArticulatedPart(next_to_attach) && !IsRearDualheaded(next_to_attach)) {

				/* Back up and clear the first_engine data to avoid using wagon override group */

				EngineID first_engine = next_to_attach->u.rail.first_engine;

				next_to_attach->u.rail.first_engine = INVALID_ENGINE;

				uint16 callback = GetVehicleCallbackParent(CBID_TRAIN_ALLOW_WAGON_ATTACH, 0, 0, dst_head->engine_type, next_to_attach, dst_head);

				/* Restore original first_engine data */

				next_to_attach->u.rail.first_engine = first_engine;

				if (callback != CALLBACK_FAILED) {

					StringID error = STR_NULL;

					if (callback == 0xFD) error = STR_INCOMPATIBLE_RAIL_TYPES;

					if (callback < 0xFD) error = GetGRFStringID(GetEngineGRFID(dst_head->engine_type), 0xD000 + callback);

					if (error != STR_NULL) {

						/*

						 * The attaching is not allowed. In this case 'next_to_attach'

						 * can contain some vehicles of the 'source' and the destination

						 * train can have some too. We 'just' add the to-be added wagons

						 * to the chain and then split it where it was previously

						 * separated, i.e. the tail of the original destination train.

						 * Furthermore the 'previous' link of the original source vehicle needs

						 * to be restored, otherwise the train goes missing in the depot.

						 */

						dst_tail->SetNext(next_to_attach);

						orig_tail->SetNext(NULL);

						if (src_previous != NULL) src_previous->SetNext(src);

						return_cmd_error(error);

					}