There is a command on the DLS called InstantLoad, and it's counterpart, InstantUnload. These do what they say on the tin. 
BEWARE: They load ALL queues on the train, so if you have, for instance, double-stack well cars with the opportunity to load different types of containers, it will load all of them, so the containers may look a little strange as the graphics card tries to decide which one should be on top. Males the train heavier, too.