if (HASBITS(v->vehstatus, VS_CRASHED)) return CMD_ERROR;

	bool free_wagon = false;

	if (v->type == VEH_TRAIN) {

		if (IsArticulatedPart(v) || IsRearDualheaded(v)) return CMD_ERROR;

		free_wagon = !IsFrontEngine(v);

		if (free_wagon && IsFrontEngine(v->First())) return CMD_ERROR;

	} else {

		if (!v->IsPrimaryVehicle()) return CMD_ERROR;

	}

	const Company *c = GetCompany(_current_company);

	/* Test whether any replacement is set, before issuing a whole lot of commands that would end in nothing changed */

	Vehicle *w = v;

	bool any_replacements = false;

	while (w != NULL && !any_replacements) {

		any_replacements = (GetNewEngineType(w, c) != INVALID_ENGINE);

		w = (!free_wagon && w->type == VEH_TRAIN ? GetNextUnit(w) : NULL);

	}

	if (any_replacements) {

		bool was_stopped = free_wagon || ((v->vehstatus & VS_STOPPED) != 0);