With the way train entities work in Source, there will be a lot of jank with rideable platforms, most notably players sometimes clipping into them and sometimes getting stuck inside them.
You still CAN do this, just parent the trailing cars to the main payload train entity, if you intend for it to travel in a straight line. If you intend to have turns and such, then it gets a little tricky. What I would do is if, say you have three moving trains, have three path_tracks to start off with in a row, with the lead train in front at the first path_track, and the trailers starting at their own individual path_tracks. Have the trailing cars start/stop as the main payload train would when players activate/deactivate the triggers. You'll likely have to fiddle around with the fine tuning settings (distance between wheels, where the origin is, etc), but it can be done. Just.... not very well, imo.
