French Travelling Post Offices

I read some french and was looking at a book where it mentioned "bureaux ambulantes." These I believe are cars at the back of a train where mail is sorted while being transported from town to town. But then I can across the phrase "courriers, convoyeurs-lignes" and these don't appear to be the same thing ... but what are they?
I readily acknowledge it is my poor French that is the problem, but the dictionary offers me little solace. I have a philatelic dictionary with french terms, "Philatelistisches Wörterbuch in sechs Sprachen" and although it is quite comprehensive it skips this one. Any suggestions on a better reference that I could acquire?

Unable to assist, this may be of interest.

A lovely book, to be sure. Not just great art came from Montmartre. The cancels depicted are all of the form town1-A-town2 so it is very similar to what I would know as a TPO. But then it is just such a different phrase to "bureaux ambulantes". I am sure it is simple, if only I could read enough to get the gist of it.

Try Googling this one,
Win10 will not allow me a link for some reason, it just downloads.

France-Rail_Mail_by_Steve_Ellis%20(1).pdf

Bingo! Here is a quote from the pdf:
"These were post office subagents who were accommodated in a small compartment of a second class carriage, working generally on branch lines or lines of secondary importance. Courrier-convoyeurs collected mail en route and deposited it at the terminus post office."
That explains it perfectly. I would never have guessed.
Not only that, but as a bonus it is a lovely display to look at!

Thanks for the link. I looked, of course, but didn't find that one.