The British Rail Class 29 consisted of 20 diesel locomotives which were rebuilt from the North BritishClass 21. The original licence-built MAN engines of the Class 21s had proved unreliable, so in 1963 locomotive D6123 was sent to Paxman'sColchester works to receive a new Paxman VenturaV12 engine. A further 19 were re-engined in 1964–1965 at Polmadie Works, with other changes made at the same time. This included the fitting of four-character headcode displays in the nose ends. After rebuilding, they returned to service from Eastfield depot in Glasgow.
Although these offered more power and much improved reliability over the original Class 21s, they did not survive much longer, due to their small class size and the use of a non-standard high-speed diesel engine. D6108 was withdrawn in May 1969 and scrapped by McWilliams of Shettleston in 1971, while the other 19 were withdrawn between April and December 1971 and scrapped at BR's Glasgow Works in 1971–72. No Class 21 or Class 29 locomotives survive today.