You treat it exactly as you would an engine cab i.e. you need the attachment point a.cabfront, an interior mesh and then add a.outsideviewx where x is a value from 0 upwards. You do not need to create an interior mesh but use either Slugsmasher's invisicab or Bendorsey's instruction sheet cab. This is what I do and just add the a.outsideview points as needed. You will find information on this in the Trainz Classic CCG (and presumably the Wiki if you have the patience)
If you want to, you can of course create an interior mesh but remember it will not show up the passengers.