What is a Commonwealth bogie?

Bogies are a chassis (or framework) that carry a wheelset on a railway locomotive or rollingstock. The Commonwealth bogie was introduced in the 1950s and used on BR Mk1s and Irish CIE Park Royal coaches. The Commonwealth bogie was manufactured under licence here in the UK by the English Steel Corporation, but the original design belonged to an American company called the Commonwealth Steel Company.

The main advantage of the new Commonwealth bogie over earlier designs was the introduction of sealed roller bearings that did away with the need to keep oil box levels topped up. Another advantage was the replacement of leaf-springs with new coiled ones that gave a superior ride quality and a speed rating of 100mph.

Despite their success, commonwealth bogies were made from cast steel and weren't very light, weighing in at around 6.5 long tons they were replaced less than 10 years after production began with the much lighter B4 bogies made from fabricated steel.