Pete;
I can't tell you specifics about the CP fleet, but I can tell you that
ore and other raw materials had vastly different unit weights, causing
those RRs hauling a variety to go through a lot of modifications to
accommodate the loads.

I know of numerous instances where the RRs tried to adjust the size of
the load, as loaded by the customer, by use of loading instruction
plaques riveted to the sides of the cars, as a way of indicating to what
depth a car was capable of being loaded, before the weight capacity was
exceeded. These were mounted onto many covered hoppers, as they tended
to carry a wider variety of load types than were placed in open hoppers.

Some RRs installed side extensions, such as the coke "racks" installed
by many on gons or early hoppers. Others built ore cars of considerably
smaller volumetric capacity to haul the heavy granular ores. A problem
was introduced when several big operations went to pelletized ores. The
smaller ore cars produced to haul only granular ore were now too small
to reach their weight capacity when loaded with pellets. The PRRs
solution was to simply build larger cars (volumetrically) and let the
old ones run at less than weight capacity. Others appear to have added
extensions.

Your hypothesis that ore produced at a certain location might be lighter
is worth pursuing. Apparently ores produced at different areas of the
country DID have different weights per cubic foot, so this would not be
far-fetched.

I have several samples of ore, including Venezuelan, Mesabi Range, and
pellets. Each has a different unit weight. I also have notes regarding
several photos of ore cars on the PRR that are loaded to different
depths. Those with the bright orange granular ore seem to be loaded to
the shallowest depth in photos.

Do you also know anything about the CP or CN loading some type of ore
(Chromium? Molybdenum? Manganese? I don't know), one scoop over each
truck, into old wooden boxcars, for shipment over the border into the
US? One of our PRR guys tells us that one of the worst wrecks he worked
was a string of Canadian boxcars that derailed and went to pieces, that
were loaded with VERY heavy ore. Why they didn't use hoppers, I don't
know, either, but were there Canadian ores that required protection
against rain? Or created some type of reaction when wet?

Let us know what you find out.

Elden Gatwood