The first of these large engines, which had a 2-10-4 wheel arrangement, was built in July 1929. Altogether, twenty were constructed before the end of this year bearing numbers 5900 to 5919. The Canadian Pacific Railway's classification was T1a. These locomotives weighed fully loaded. They were the largest and most powerful non-articulated locomotives in the British Empire. They were named after the Selkirk range of mountains that they crossed, the (railway) summit of which was located just inside the western portal of the Connaught Tunnel beneath Rogers Pass.