Canadian Mobile Post Office Of Some Sort?

This cover went from Crouseville, Maine, south to Houlton, then across the border to Woodstock, New Brunswick (Houlton and Woodstock were exchange offices) then north to Victoria Corner, Carleton County. The part I don't understand is the Vanceboro & Andover / P. C. postmark. Vanceboro is in Maine, on the border, near McAdam, or south of Woodstock, Andover is now part of Perth-Andover, north of Victoria Corner. A route that the letter would have followed part of, but what was it? Perhaps something comparable to a pre-RPO Route Agent? What did P. C. stand for? Did that route, whatever it was, actually cross the border into Vanceboro, Maine?

For that matter, what was the system for exchange offices that were ten miles apart? Did two mail carriers meet at the border? Did they carry mail directly from one post office to the other post office?

My thought- I am thinking " mail car or motor coach" but not at all sure. Was M.C. used in the U.S.

Vanceboro & Andover P.C. (Postal Car) is listed in the Maritime RPO section of the 'Catalogue of Canadian Railway Cancellations' and operated from 1888-1903.

Thank you! I found the BNAPS RPO Study Group webpage, which includes a spreadsheet of Canadian RPOs.

I still don't know quite how things worked with cross-border RPOs, though. Canada seems to have had a number of them, while the US seems to have had very few.