Modifications led the T1b to be ten tons or tonnes lighter while increasing steam pressure from 275 to 285 pounds per square inch (1.90 to 1.97 MPa).

All of the later T1b and T1c-class locomotives were semi-streamlined and were painted with CPR Tuscan red panels along the sides of their running boards, beneath the cab windows and on the sides of the tenders with gold leaf (originally) border trim (which was later changed to a type of yellow similar to "duluxe gold").

Some of the original T1a-class Selkirks also received this paint scheme (but with narrower panels along the sides of the running boards that widened in the middle to carry the locomotive's road number).

All 36 Selkirk locomotives (5900 through 5935) were initially assigned to handle both freight as well as passenger trains between the major division points of Calgary and Revelstoke, a distance of 262 miles (422 km).

They were also used for 23 miles (37 km) in pusher service from Revelstoke west (uphill) to the wye at Taft, assisting the road engines of both freight as well as passenger trains up the steep grade to this location.

The Laggan Subdivision covered the eastern portion of their assigned territory 137 miles (220 km) and included the Field Hill and the Spiral Tunnels while the western portion of their primary, assigned territory consisted of the Mountain Subdivision which covered the remaining 126 miles (203 km) and included Rogers Pass, the Connaught Tunnel and Stoney Creek Bridge.

The booster engine added an additional 12,000 lbf (53 kN) of tractive effort up to a maximum speed of 20 mph (32 km/h).