The CPR was building steel cars in the mid-teens, although one of the most common cars in your era would have been the Dominion Fowler Patent boxcars. The CPR owned thousands of them, mostly 36-footers. The cars were singlesheathed, with an exterior framework of "Zed" bars (that's Zee bars to our American friends

) These cars saw service at least into the 1960's, especially on light rail prairie branches. Proto1000 makes a model of the cars, in CNR, CPR, and TH&B paint schemes. If you can live with the oversize plastic grabirons, these are an otherwise excellent representation of the prototype. Here's a photo of one of mine; the grabirons and steps have been replaced with metal parts.
The CPR also had quite a few of these cars with drop doors in the floor, and some with hopper bottoms, for coal and grain service. The Proto1000 cars are a bit pricey, at between $35.95 and $39.95 around here. They also make, in the same roadnames, 36' Fowler Patent stockcars, offered at a similar price.
Don't forget that the CPR was connected, through interchange, with all other common carrier roads in North America, so you could conceiveably have just about any car that was running in that era show up in your neck of the woods. Lots of the bigger roads started building steel cars in the early 20th century, in particular the NYC and PRR. Any of the USRA doublesheathed cars, as offered by Accurail, would also be appropriate, along with their 6- and 9-panel singlesheathed boxcars.
Wayne