Having an unloader/loader building would make this much better for automation:

e.g. you drive the tank to a drop off point which uses conveyors to move the tank to a train, which can then carry it back to base, refill the inventory, and ferry it back to be unloaded

(assuming you can get graphics, I'd make them act like inserters, so you can rotate them for load/unload direction and or circuit net connections)

I like this idea a lot. When I started playing with this mod today, I expected the winch to be a building, actually. Clicking stuff seems somewhat un-Factorio.

The biggest challenge might not be graphics, but animations. Having vehicles teleport onto the train would look pretty strange.