I'd like to expand a bit on Ed's perceptive remarks and question. My own experience and researches agree with his comment about the high proportion of home-road cars in Canada.

The model for proportions of "foreign" road boxcars that has been extensively discussed on this list probably does not apply for Canada in the time period under discussion. US boxcars and Canadian "other road" boxcars were seen in Canadian freight trains, but in most cases probably less than the US "proportion of fleet" model would suggest. I know this is a gross generalization, and observations in the east would differ considerably from those in the west, by time of year, etc.

We can speculate why this might be. On the use of home-road cars in Canada, the two big Canadian roads were both truly transcontinental, and their service areas considerably overlapped. As a result, for shipments within Canada one could expect a higher proportion than in the US would see origin and destination both served by the same road. Supporting this inference about reduced interchange requirements is the surprisingly high proportion of Canadian cars in service in the period that were not legal for interchange - for example, something like half of all CPR flat cars still had K brakes well after the cut-off date, so could not be interchanged with other roads (straying a bit from boxcars with that example.)

While Canada and the US were indeed allies in WW2 (though Canada had been at war for a couple of years before the US joined in as one of the allies) the economies of North America were far less integrated in those days than one might expect. For many commercial and industrial relationships, Canada was still more closely tied to the UK. Trade tariffs and other barriers were still fairly strong at the US-Canada border, and much less moved back and forth than would later occur with the Auto Pact, consolidation of production and distribution plants and ultimately the FTA and NAFTA.

That's not to say that Canada-US trade wasn't important at the time, but it does help explain why Canadian cars seemed to be under-represented in the US. CNR and CPR had boxcar fleets, for example, of sizes that would put them amongst some of the biggest of the US roads (PRR, NYC, ATSF, SP, UP) but would have been rather more rarely seen in the US, except at industries regularly receiving Canadian goods such as newsprint or other forest products. The same logic suggests why US boxcars would be relatively rarer in freight trains within Canada (except those in transit between two US points, such as from Chicago or Detroit across southern Ontario to Buffalo, that others have previously mentioned.)

Hope that's helpful, perhaps others have some further insights?

Doug Rhodes