If you just want a transport to take units from one factory to another point on the map. Set a rally point for the factory then tell the transport to 'assist' the factory. All units load into the transports that automatically deliver and return, any units made while the transport is full wait outside the factory until a transport returns. If a backlog occurs. Get another transport and tell it to assist the factory until there is no backlog, then just leave it running. You'd be surprised how fast you can get a couple of hundred light vehicles accross a large map using 2-3 factories churning out tech 1 tanks with a pair of large transports assisting each factory.