Bogie bolster wagons were used for carrying long loads, such as girders, rails and complete sections of track. Their open bed made for easy loading and unloading and the load was held in place by vertical steel pins pushed into wooden or steel bolsters set across the floor of the wagon, at intervals along its length. For wider loads, pins could be set into pockets at intervals along the sides of the wagon.

The 'Macaw B' was pretty much the GWR's standard bogie bolster wagon and had a long life in production. They were being built before the First World War, some being converted to tank carriers for use in France. There were many modifications down the years. The LMS adopted a Midland Railway design for their 30 ton bogie bolsters and between 1926 and 1934, built 327 of them. British Railways adopted more than one design from the former railway companies and, for its 30 ton 'Bogie C' wagons turned to the GWR 'Macaw B' design which had four moveable bolsters. Between 1949 and 1962, many batches of these were built.