The British Rail Class 144 Pacer are diesel multiple unit (DMU) passenger trains built at Derby between 1986 and 1987.

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.

[9] 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.

[6] 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.

[6] 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.

[6] During this period, a number of prototypes were constructed to explore different designs and approaches for implementing the railbus concept.

[10] Initial testing with the Class 140 uncovered several issues, such as difficulty detecting the type via track circuits, this was reliably resolved by swapping the material of the brake blocks from a composite to iron.

[8] The units have a maximum speed of 75 mph (121 km/h) and are externally similar to the earlier Class 143 Pacers, sharing a near-identical body built by Walter Alexander.

Individual vehicles are numbered as follows:[1] As originally built, the traction arrangement of the Class 142 consisted of a 200 horsepower (150 kW) Leyland TL11 engine, a Self-Changing Gears mechanical automatic gearbox and a Gmeinder final drive unit on each car driving only a single axle.

[6] Starting in the early 1990s, every member of the class was mechanically upgraded due to the original parts being determined to be a major source of unreliability.

[11] The most substantial change was the replacement of the Leyland-built engine with a more powerful Cummins LTA10-R[3] as well as the original mechanical transmission being substituted for a Voith hydrokinetic unit.

[11] They were produced specifically to perform local services sponsored by the West Yorkshire Passenger Transport Executive (since rebranded as Metro).

[citation needed] Early operations of the type were marred by unreliability to the extent where the Class 144 was commonly substituted for at the last minute by either first generation DMUs or locomotive-hauled trains.

According to industry periodical Rail magazine, the Class 144's mechanical transmission was frequently attributed as being the largely single cause of these issues, while undesirable instances of wheel slippage was another common occurrence.

This refit not only resolved much of the unreliability issues but also gave the Pacer greater acceleration through the installation of a more powerful engine.

[21] Following the outbreak of COVID-19 in the UK in March 2020 and the resulting curtailment of passenger services,[22] Northern withdrew the entire fleet prematurely and by mid-April had placed all 23 units in storage; 18 units at the Keighley and Worth Valley Railway and the remaining five at Heaton Traction Maintenance Depot in Newcastle upon Tyne.