I suppose I would kitbash Johnstown cars out of the sides of some heavyweight passenger cars, and a simple peaked roof. The ends are open with just some pantograph gates. For the Duquesne incline, I would use The sides of and "old time" wooden combine to get the large side doors, and the siding texture. No end doors.
The chassis would be made after determining the angle of the plane. Just 2 axles, and I would consider Std Guage 'guage' for o scale to represent the wide guage track.
Movement is simple as the cars counterbalance off of a single axle. This is easy to see if you visit the Pittsburgh or Johnstown inclines, as they have windows into the motor room.
I seem to recall seeing a picture of an incline on Jim DeSantis' amazing tinplate layout. Cars were cut down observation cars. https://ogrforum.ogaugerr.com/t...ard-gauge-collection
