The British Rail Class 144 Pacer was a diesel multiple unit (DMU) passenger train built at Derby between 1986 and 1987. BR, 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 15 units are in operational condition. Background By the beginning of the 1980s, British Rail (BR) operated a large fleet of first generation DMUs, which had been constructed in prior decades to various designs. While formulating its long-term strategy for this sector of its operations, British Rail planners recognised that there would be considerable costs incurred by undertaking refurbishment programmes necessary for the continued use of these aging multiple units, particularly due to the necessity of handling and removing hazardous materials such as asbestos. In light of the high costs involved in retention, planners examined the prospects for the development and introduction of a new generation of DMUs to succeed the first generation. In the concept stage, two separate approaches were devised, one involving a so-called railbus that prioritised the minimisation of both initial (procurement) and ongoing (maintenance & operational) costs, while the second was a more substantial DMU that could deliver superior performance than the existing fleet, particularly when it came to long-distance services. While the more ambitious latter requirement would ultimately lead to the development of the British Rail Class 150 and the wider Sprinter family of DMUs, BR officials recognised that a cheaper unit was desirable for service on the smaller branch lines that would not be unduly impacted by lower performance specs or a high density configuration. As such, work to progress both approaches was undertaken by BR's research department during the early 1980s. During this period, a number of prototypes were constructed to explore different designs and approaches for implementing the railbus concept. One such vehicle was a single two-car unit, designated as the Class 140, that was constructed between 1979 and 1981. This prototype was introduced with much fanfare during June 1981. Initial testing with the Class