In 1929 Canadian Pacific received the first of 7,500 all steel 40' box cars. By the time the deliveries were completed in 1931, they totaled 11% of their box and auto car fleet. The standard CP box car prior to their delivery was the 36' Fowler-patent single-sheathed box car.

 

While the dimensions were similar to the 1923 ARA recommended practice/PRR X29 box car, there were significant differences in the construction of the car. The most significant of these was the 'tabbed' side sill and the attachment of the steel sheathing. The X29 type box car had a tendency to trap water at the side sill, rusting out the side sheathing at that point. Evidence of this are the many photos of these cars in later years with patched side sheathing.

 

The sides of the 'Minibox' are attached to a steel angle that is attached to the top of the sections of channel that make up the 'tabbed' side sill.This innovation is part of what gives these box cars their unique look. The resemble short versions of the 1932 ARA standard steel box car. This is the first known application of this type of attachment of the sheathing to the side sill. Although there is no direct evidence, it appears this innovation directly influenced the design of the 1932 ARA standard.

The cars were very successful, with almost 6,400 still in service in 1967, over 1,800 in 1972, and one still reported in revenue service through 1983. Cars were used in MOW service through at least 1993. Some of the cars in work service were modified with tall steel frames on the roof for icicle breaker service west of Calgary.

The British Columbia Railway (BCOL) acquired several ex-CP cars for use in MoW service.