Postboxes and mail on trains

Hi

You might or might not know that I collect postboxes. A few weeks ago I visited a friend who has a large collection of French postboxes including some that were used on trains. 

I actually bought one from the 1930's. Its a small box that has a frame that I believe was attached to the side of the carriage and then at various stations the box would be removed from the frame by station staff and replaced with an empty box. 

So I have two questions;

1. Does anyone know if mail posted on french trains received special postmarks?
2. Does anyone know any more about postboxes on french trains?


Thanks in advance

Alex