Yeah your right in that it would need to be an x file then use entity workshop v2 or entity maker or a myriad of other programs to import it to fpsc.
As for the scale fpsc is weird. using google sketchup in metres to make my models I noticed they were tiny when imported to fpsc, so for scaling purposes I found a site online that you could upload x files to and download as collada or 3ds or whatever you wanted it converted to, so I took a human figure from an fpsc model pack, converted it to a format I could import to sketch up, and everytime I made a model I imported the figure and scaled my model to match.
thus far is the only sure fire way of scaling items i have found, visually.
I did the same with floor and wall segments so I could model in sketchup directly onto an accurately scaled fpsc segment so I knew my models size and scale would be correct.
So my recommendation would be to do the same, model the subway car then convert and import your placeholder and scale your subway car to match inside the modelling progam you are using then export it as an x file load it up in entity workshop, texture and export it directly to fpsc.

hope that helped
smoke em if you got em