TPO -> Canada RPOs
Canada RPOs
By Tony Goodbody
Canada is a large country for the most part sparsely populated. Nevertheless there are nearly 2500 known RPO postmarks with a further 150 from Newfoundland and 250 maritime postmarks.
(N.B. any postmark with the initials 'TPO' is almost certainly a maritime postmark unless the word 'Railway' or an abbreviation of it appears in the postmark. For this reason the designation RPO will be used to denote a railway travelling post office.)
Early postmarks, from 1853, tended to be large and attractive, incorporating the route and/ or the name of the railway. Of such marks it is very difficult to find good strikes.

Shown above is a cover addressed to Ottawa with the postmark reading MONTREAL & ISLAND POND G.T. R. WAY/ A/ UP/ JAN 1/ 1859/ No. 7. The cover also has a rate mark '3' almost certainly applied by the RPO. The railway was the Grand Trunk.
By the 1890s and possibly earlier the large marks began to disappear in favour of much smaller marks with the designation M.C. (Mail Car).
It will be noticed that the postmarks often include the route in abbreviated form. Thus "TOR & SAR M.C." is the Toronto & Sarnia Mail Car." Similarly "CH'TOWN & TIGNISH" is Charlottetown & Tignish M.C.

In time the designation "M.C." disappeared in favour of "R.P.O." The cover above shows the ST. STEPHEN & EDMONSTON R.P.O. with the direction indication 'S'.

Occasionally "RPO" was replaced by "Ry.P.O."
On busier routes with several cancellers we may find a number at the base of the postmark and the train number above the date.
Travelling Letter Boxes
Here is a cover from Midland, Ontario with postmark reading "THIS MAIL WAS CARRIED BY TRAVELLING LETTER BOX TO TORONTO." The route from Midland to Toronto was covered by the TORONTO & MIDLAND RPO and it is not clear why the travelling letter box was employed.

THIS MAIL WAS CARRIED BY TRAVELLING LETTER BOX TO TORONTO
Clerk's Handstamps

Rare mark: "ROUSES PT. & MONTREAL RPO" with the date May 27, 1914.
In Canada and the U.S.A. the RPO clerks often had their own personal handstamps. Very occasionally covers may be found with the stamp cancelled by such a private handstamp.
Bibliography:
L.F. Gillam, "A History of Canadian RPOs 1853-1967" published by the American Philatelic Society, 1967.
Lewis M. Ludlow, "The Catalogue Section of the Handbook of Canadian Transportation Postmarks" published privately in the U.S.A. in 1982.