Lakie, I can only comment on what I know from researching trains in the UK and Queensland.
Firstly, locomotives often have different liveries depending on whether they are mainly used for passenger services, or mainly used for freight/mixed traffic. This is especially the case with steam locomotives, but also applies to diesels and electrics.
However, I think that these differences can be made by the graphics creator, rather than needing explicitly different colour schemes.
Secondly, the passenger/freight wagon livery is completely unrelated to the locomotive livery (eg, in the 1960s in the UK, the diesel locomotive livery was dark green, passenger carriages were red and cream, and freight wagons were brown - in the same period in Queensland, diesels were blue, passenger cars were red, and wagons were black). Obviously this doesn't apply to MUs, special train sets etc - this can probably be got around in TTDP by making wagons take the colour scheme of the locomotive if a wagon override is used?
Also, MUs (in TTD terms, any train which carries passengers/mail in the "locomotive") often have a completely different livery from either regular Locomotives, or from locomotive-hauled passenger cars.
So my personal "wishlist" of options would be
- Steam loco
- Diesel loco
- Electric loco
- Passenger wagon
- Freight wagon
(where "Passenger wagon" includes mail vans, etc)
Do monorails and maglevs need seperate colours for their wagons? Probably not.
There was another idea I had, but I'll save that for IRC.. I don't want to get people excited in case it turns out to be not possible.
