Bogies are an independent framework that supports the wheelsets on a railway locomotive, coach or some larger wagons. The B4 bogie was a type introduced in 1963 to replace the earlier Commonwealth bogie design. The B4 weighs in at just 5 long tons thanks to its fabricated steel design making it 1.5 long tons lighter than its cast-steel predecessor, another advancement was the introduction of double coil-springs per wheel compared to the single spring design of the Commonwealth bogie.
The B4 bogie was introduced in time for the production of the Mk2 coach on which they were fitted from new. They were also retrofitted to some Mk1 coaches and a heavier version classed as a B5 bogie was produced for Mk1 based Southern EMUs and the heavy end only (kitchen end) of some restaurant coaches. The B4 had a speed rating of 100mph, however, some were uprated to 110mph subject to more stringent maintenance schedules.