So I tried interiors,
Sorry to quote pics, but its a start, its a box on wheels, but its a start. One tip though, learn splines and use the extrude modifier from that, because you can make some interesting carriage sides out of it.
http://www.tutorialized.com/view/tutorial/3dsmax-studio-spline-modeling-tutorial/37641
Another thing, make sure your stock is 1.0m off the ground, the floor level between 1.15m to 1.27m off the ground (IIRC), the height is no higher than 3.77m and the bogie attachment points around about 0.468m (for the BT13s) and I think, 0.5m for the gresley bogies, again, off the ground/rail level. If you are doing it in max units, remove the m.
Interiors are not hard to grasp, try adding a couple of a.outsideview0/1 whatever points and have a interior line in the config file, you get some interesting views.
The other thing, please, avoid PNG's! It takes forever to download even on my connection! Convert them to JPGs first, then upload. Oh, don't upload bmps either, ever, they are uncompressed files. (Internet Explorer has a habit of handling PNG's badly)
EDIT:
2 other things I wanted to point out, seats, measure your desk one, it should be about 40-50cm off the ground, try 40cm for the bottom to see what happens. Obviously, the slimmer the material, the higher the seat will be off the ground.
Secondly, try checking your measurements against these (
http://www.btinternet.com/~joyce.whitchurch/gauges/text.htm). If you need a quick way to check, copy the C3 one, create a plane thats 3.77 in height and 2.62m across for 23m long carriage and 2.82m across for 20m long carriages, then put it smack bang in the centre of the carriage, to see if it fits. If it doesn't, you're doing it wrong!