Function CheckNewTrain

src/train_cmd.cpp:992–1009

* Check/validate whether we may actually build a new train. * @note All vehicles are/were 'heads' of their chains. * @param original_dst The original destination chain. * @param dst The destination chain after constructing the train. * @param original_src The original source chain. * @param src The source chain after constructing the train. * @return possible error of this

 * @return possible error of this command.
 */
static CommandCost CheckNewTrain(Train *original_dst, Train *dst, Train *original_src, Train *src)
{
	/* Just add 'new' engines and subtract the original ones.
	 * If that's less than or equal to 0 we can be sure we did
	 * not add any engines (read: trains) along the way. */
	if ((src          != nullptr && src->IsEngine()          ? 1 : 0) +
			(dst          != nullptr && dst->IsEngine()          ? 1 : 0) -
			(original_src != nullptr && original_src->IsEngine() ? 1 : 0) -
			(original_dst != nullptr && original_dst->IsEngine() ? 1 : 0) <= 0) {
		return CommandCost();
	}

	/* Get a free unit number and check whether it's within the bounds.
	 * There will always be a maximum of one new train. */
	if (GetFreeUnitNumber(VEH_TRAIN) <= _settings_game.vehicle.max_trains) return CommandCost();

	return CommandCost(STR_ERROR_TOO_MANY_VEHICLES_IN_GAME);
}

/**
 * Check whether the train parts can be attached.