For the production line in the previous problem, assume that a single AGV train consisting of a tractor and multiple trailers are used to make deliveries rather than separate vehicles. Time required to load a pallet onto a trailer = 15 sec; and the time to release a loaded train and move an empty train into position for loading at the end of the production line = 30 sec. The same times apply for pallet handling and release/positioning at the unload station located at the front of the production line. If each trailer is capable of carrying four pallets, how many trailers should be included in the train?