Pacer was the operational name of the British Rail Classes 140, 141, 142, 143 and 144 diesel multiple unit railbuses, built between 1980 and 1987. They were inexpensively developed using a passenger body based on the Leyland National bus on top of a chassis based on the HSFV1 research vehicle. The railbuses were intended as a short-term solution to a shortage of rolling stock, with a lifespan of no more than 20 years. As modernised replacements were lacking, the Pacer fleet remained in service on some lines until 2021 – 37 years after their introduction in 1984.

All Pacer trains were scheduled to be retired by the end of 2019 as the PRM-TSI requires that all public passenger trains must be accessible to disabled people by 2020 – however the Pacer units were given dispensation until the end of 2020. Only one Pacer (the modernised 144e) met this requirement, and the remainder were therefore planned to be withdrawn by that date. Furthermore, a decision in 2015 by the Transport Secretary required that such railbuses be removed from service by 2020 for the then-new Northern franchise, stating that the "continued use of these uncomfortable and low-quality vehicles is not compatible with our vision for economic growth and prosperity in the north".

At the start of 2020, 138 Pacer units of classes 142, 143 and 144 were either still in service or storage with three National Rail operators: Arriva Rail North, Great Western Railway and KeolisAmey Wales. After the 144s were withdrawn from the Northern franchise, Northern Trains retired its last Pacer unit, a 142, on 27 November 2020, with Great Western Railway following with its last 143 in December 2020. KeolisAmey Wales withdrew only its 142s in late 2020, passing its 143s to Transport for Wales Rail who finally phased them out in May/June 2021.