1) I definitely feel that it needs to be possible to put mailcars on all trains, as we have decided.
2) Right now, we have an interesting mix: some trains have very clearly different mail and passenger cars, some have mail cars that are identical and indistinguishable from passenger cars, and some have mail cars that look very very similar but not quite exactly the same as the matching passenger cars. Personally, I prefer the last option: each train has mail cars that are in the same livery, but basically just have some of the windows blacked out. That way you can tell that they're there if you are looking for them, but they don't smack you in the face as an obvious mismatch. The only downside I see to that is that if there is a mail car as the very last car, you don't get the appropriate tailcar. But I bet there's a way to fix that via GRF: can you just have mail cars always shoved to the front of the train?
3) Also, I like the push-pull plan for MUs. It's true that the mailcars will "jump"; but if they are drawn as designed above, and the pantographs are coded so they do NOT jump, then it will not look odd unless you are looking closely to catch it I think. We might want to test this first though.
4) I am 100% in favor of having the MUs gain in running cost with the length of the train. That way we can make them quite cheap - but the costs add up as the train gets longer, until at some point you might be better off just buying a normal loco and unpowered cars. Which is the way it is in real life too.
