I've delt with this problem by modifying the congif file for the train cars. I put the commodity I want to instant load as the first commodity in the first queue. I have to keep track of which car is set to haul which commodity, but so far it is working for me.
I too am interested in continuious operations. To that end, I have created the CMTM System (Car Movement & Traffic Management). When a session is begun, every car on the route is set to an empty or a load and is given an off route destination. For all trains entering the route via a CMTM portal, each freight car is given a destination and is set to loaded or unloaded status. No interactive industries are required, although I like to have those that deal with visible loads to be Proto-Lars for the animation of loading/unloadiing.
Features I am currently adding are the ability to load specific commodities into any car that is set-up to handle that class of commodity and, contunious 24/7 operation with the ability to jump time to the next active period for your route.
Dap