There are several limitations to Canadian slab-side covered hopper circulation in the states.  

(1).   Cement and lime tend to be low-value, short-distance hauls for any covered hoppers in this trade.   The raw materials and cement plants are commonplace across North America, no need to sell limestone, lime, or cement far away.   So these Canadian covered hopper cars might be seen aplenty in US border states yet no farther.

(2).   These cars were few before 1956 or so.   Railroad managements clung to multi-use general freight cars for their backhaul possibilities, and shunned specialty freight cars for their inherently empty return trip.   Only when that last management generation was consumed ... in the biblical sense ... did specialty freight cars blossom.

(3).   To use covered hoppers, both the shipper and consignee must have proper operational facilities to load and unload, respectively.  No mechanical bull bucking bronco boxcar unloaders permitted.   In our modern experience, covered hopper loading/unloading is a slam dunk, but in the 1950s and 1960s, this was new materials handling technology.    Customers facilities and covered hopper fleets grew together, slowly at first, and could constrain one another.

(4).   Canadian freight cars could only ride American rails for 30 days before being dubbed as sales by customs.  This put all Canadian freight cars on a short leash in the USA until the NAFTA accords in 1993 or so.  And vice versa.   To me, this was stupid, but customs inspectors are from the government and here to help us.


Another prospective load was specialty clays.   Along the TH&B, General Refactories in Maitland, Ont. manufactured high performance fire brick, such as for blast furnaces.  Clays, dry chemical additives, chromate smeltings, trace additives, and other ingredients were shipped to the plant from various sources, including in the US.  B&O and TH&B participated in a covered hopper pool, one could see several loaded cars in a string headed north from Pittsburgh.   Origination is vague, surviving documentation is incomplete: my 1963-64 Buffalo Division northbound wheel reports do not list source, southbound wheel reports showing empties went into some other unwatched dumpster far, far away.

Bruce, this may only tangentially answer your question but I found a picture of CP XX0294 on the GN in Edmonds WA in July 1960 in Hickcox' book Great Northern in Color, volume 1, which was enough to justify my buying one of Sylvan's slabsides and modeling the CP car.  Understand that CP tended to interchange more with NP, while CN's favored interchange partner for US destinations (at New Westminster BC) was GN.  So if a CP car showed up on the GN, it is highly likely that CN slabsides also made it to the US.  A lot of CN cars traveled via the GN over the Inside Gateway (GN-WP-ATSF) to Southern Cal.

Doug Polinder
Seguin TX