Thanks to all for their contribution to this fabulous post.
Somehow, I never really checked my collection with RPOs in mind.

Now you people got me hooked and I will sort through all my stuff looking for them!
So first, a picture of the interior of a mail car that you can see at the Canadian Railway Museum in Saint-Constant, Quebec.
It's C.P.R. 3618, built in 1940 by Canadian Car & Foundry's Angus Shop in Montreal

Browsing through my collection quickly, I found 5 stamps that have clear enough RPO cancels to identify them.
Here they are.
Please, feel free to comment.
Bujutsu, if you have the time or the inclination, I would very much appreciate your feedback. I don't have Ludlow's Handbook for the time being (I will find a copy eventually!) and I'm dying to know more about these stamps!

1) SOURIS C.P.R. SECTION M.C. on Scott#77

2) C.N.RW'Y EMERSON & WINNIPEG R.P.O. on Scott#98

3) MED.HAT & NEL. R.P.O. on Scott#106

4) Not so sure about this one... think it's HAMILTON & PALMERSTON R.P.O. on Scott#191

5) Had to research this one a little bit, it's COR. & LAC. R.P.O., hammer no.1, on Scott#197

I can't wait to have enough time to browse more carefully through my collection!! :)