The GWR Macaw B, BR code Bogie Bolster C, is a 45-feet length flat bogie wagon designed to carry long loads bolster is a type of flat bed wagon designed for carrying long loads. This design with heavy angle trussing was designed to carry loads up to 30 tons distributed along the length of the wagon and was equipped with the GWR standard plate back bogies. This allowed a wide range of loads to be carried, from the original long lengths of timber to structural iron and steel products like beams and rail. Other items which were simply too long for a conventional 4-wheel wagon could also be carried, new ships lifeboats being a not uncommon example.
The GWR bogie bolsters continued in service with BR into the 1960s before being replaced by newer built wagons. Several examples have been preserved.