In 1937, the AAR approved a new standard box car. The design itself was an update of the 1932 ARA box car design, with increased height and width, and newer components. 

While the 1932 ARA car received some acceptance by the railroads, the 1937 AAR design was the first standard steel box car built in large quantities by many roads including Canadian National and Canadian Pacific, rostering nearly 30,000 cars between them between 1937 and 1944.

 

The Candian cars have some unique features. They were slow to adopt the new Rectangular Panel roof, instead using an earlier Flat-Panel design until as late as 1943. They used unique ladders with integral stirrup steps, including end ladders, and since they did not have poling pockets and stirrup steps, they lacked the side-sill 'tabs' at the corners of the cars. They also continued building to the 1937 AAR standard cars as late as 1944, when other roads had built 'Modified 1937 AAR' cars with an increased interior height of 10'6" starting in 1941. Both roads switched to the 1944 'Postwar' design in 1947.