would seem alot more realistic if we could have one or more types of train skins allowed to give a varied look in train stations.?
Yuu should look for train automata files in the LEX and STEX (over at simtropolis). That way you can have a greater variety of engines pulling your trains, however not every automata addon is fully compatible with other automata addons.
The game randomly chooses which car to use.
For freight, it's not really much of an issue, since mismatched/random cars are the norm.
Passenger trains, however, will mostly end up as an ugly mismatch of several liveries.