Canadian stock cars did indeed travel across the border into the U.S. for about a century transporting animals for grazing, feeding and slaughter.
A 44-page Canadian National Railways publication "Rules and Regulations governing Transport of Livestock" includes 22 pages dealing with U.S. government regulations. The imported cattle must be accompanied by various certificates signed by a Canadian veterinarian indicating they are from tuberculosis-free herds. With the required documents animal shipments would proceed without quarantine. Cattle shipments without certificates were detained in quarantine to be inspected. Also shipments of livestock entering at a Canadian port-of-entry but originating from most other foreign countries were subject to quarantine at the port-of-entry. The following states had quarantine stations for animals entering from Canada: AK(2), MT(2) OR(1), ME(15), NY(17), VT(8), WA(12), MI(3), ND(2). The place names are listed.
CP and CN 8-hatch steel reefers were used to ship perishable products in Canada for many years before the advent of mechanical reefers. Perishable products needing protection from freezing temperatures or heat constitute a long list and include fruit, vegetables, canned goods, beverages, eggs, butter, fish, sea food, and meat. It is a myth that these 8-hatch reefers were primarily used to ship meat. Any perishable products of Canada that were exported to the U.S. by rail during that era likely travelled in an 8-hatch steel reefer.