I set up a few cars on 2 stations that did NOT equal the number of cars. The train seems to favor the car directly behind the engine.
Example:
Station 1 modules (load): unit, unit, loose, liquid
Station 2 modules (unload): unit, unit, unit, unit, loose, liquid
Train 1: engine, unit, unit, unit, unit, loose, liquid
Unloads all of the cars and not the liquid.
I tried this: engine, liquid, unit, unit, unit, unit, loose
This seems to work.
If you have dedicated stations unload/load set up exactly like the train, all works but different combinations do not.
I would assume that if I set up a train that had 6 unit/loose cars (all the same cargo) and 1 tanker and I sent it to a station with 1 unit and 1 liquid, it would keep dumping all the cars in order then the tanker but it does not. The water never gets delivered.
I assume it's the same if I used all tankers and a few loose.