Even more evidence of what lousy stand ins they were.
As far as the French loco's cab, that would have been a wooden and drywall mockup anyway.
Not sure which movie you're referring to, but if you mean "The Train," I don't believe any mockups were used. They used actual SNCF locomotives, primarily 1-230.B class 4-6-0's, outfitted with cameras. Several actors, including Burt Lancaster as mentioned above, actually learned the basic operation of the locomotives for the film.
They wrecked real locomotives (they were getting old at the time), a yard, and several cameras while making the movie. For example-
-In the scene where the locomotive derails coming out of a siding, the (real) engineer brought the train out much faster than expected. When it jumped the tracks, it slid further than expected, crushing several cameras that had been set up. The only camera that survived was set at ground level along the tracks and ended up with an incredible shot of a derailed wheel spinning over it as the locomotive stops. During the initial screening, when this scene came up, an executive turned to John Frankenheimer, the director, and said something like, "Wow! That's incredible! How did you get that shot?!" He didn't tell him it was basically an accident because all the other cameras had been wrecked in the shot.
-When the railroad yard gets bombed by allied planes, that's a real yard being blown up with dynamite. The railroad wanted to reconfigure the yard, so they allowed the movie to blow it up and clear the land prior to construction.
This info can be found on the DVD commentary for the movie.