First, let me note that all the photos I reference below show cars in the green/black/gold scheme, right through 1971.
Railroad Post Office History (copyright 1972, Mobile Post Office Society) has 2 photos of CN baggage mail cars in use on the Central Vermont between St Albans and Springfield Mass from 1960-1966 (I think it may be photos of the same car, but can only read the number in one photo). The photos show a car similar to the MT car, except that the window spacing between the first and second window is wider than between the second and third, and the first is covered with some sort of screening or ventilator (maybe just a plate). Number is hard to read, but I think it is 7806. The route information shows CV serving a route from Boston to St Albans from 1901 to 1960, Brattleboro to New London from 1890-1946, Montpelier to Williamstown 1934-1938, and Montpelier & Barre from 1899-1934. Here were many other routes that last ran prior to 1918. The Springfield and St Albans route is the only one I could verify with photo evidence of a CN car.
Canadian National itself is listed as operating US Mail routes including Duluth & Fort Francis (and operated north of the border to Warroad) from 1909-1960, and in the east, from Portland (Maine) to Island Pond 1882-1934 and from Portland to Boundary Line 1934-1960.
I've checked Dorin's book on the GTW. P 48 shows a photo of CN 7810 with US Mail markings in Battle Creek in 1971 in poor repair (15' RPO/2 baggage doors per side) and notes it was used on several US mail routes. That is the only photo I can locate of a CN RPO on the GTW. It is not the same prototype, but would indicate that a CN mail-baggage car with US Mail markings would be plausible on the GTW, at least late in the RPO era. All the GTW Mail-baggage cars in my books appear to have 2 baggage doors on each side.
Here is a link to photo of 7806 that was posted by Mike Skibbe yesterday on the N Scale Varnish yahoo group- with US mail markings:
http://www.cnrphotos.com/gallery2/main.php?g2_itemId=16986Tom D
