The British Rail Class 144 Pacer is a diesel multiple unit (DMU) passenger train built at Derby between 1986 and 1987. British Rail, seeking to procure improved derivatives of the earlier Class 141, placed an order with the manufacturers British Rail Engineering Limited (BREL) and Walter Alexander to construct their own variant, the Class 144. A total of 23 units were constructed. All units have now retired from mainline service, though the majority of the units have been acquired for preservation on heritage railways and in other uses. As of December 2022, 19 out of the 23 units have been purchased following withdrawal for this purpose, of which 14 units are in operational condition.

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.