Class 142 diesel multiple unit passenger trains were built for British Rail (BR) from 1985 to 1987, with a high level of commonality with the widely-used Leyland National bus.

By the beginning of the 1980s, British Rail (BR) operated a large fleet of first-generation diesel multiple units (DMUs), which had been constructed in prior decades to various designs.

[11] While formulating its long-term strategy for this sector of its operations, British Rail planners recognised that considerable costs would be incurred by undertaking refurbishment programmes necessary for the continued use of these ageing multiple units, particularly due to the necessity of handling and removing hazardous materials such as asbestos.

[9] 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 better performance than the existing fleet, particularly when it came to long-distance services.

[9] While the more ambitious latter requirement would ultimately lead to the development of the British Rail Class 151 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.

The underframe area, in addition to its structural role, accommodated all of the propulsion apparatus along with the majority of electrical gear.

[9] Unlike the Class 141, a microprocessor-based controller for the automatic transmission was used from the onset, allowing the reliability issues posed by defective relay logic and poor earthing present on the predecessor to be entirely avoided.

[15] Both axles (one driving per coach at the inner end) were fitted directly to the chassis rather than being mounted on bogies, unlike traditional DMUs.

This uncommon arrangement has been attributed with resulting in the Class 141 units possessing a relatively rough ride, especially when traversing jointed track or points.

[9] Excessive flange squeal on tight curves has been a problem on many routes operated by the Class 142,[17] caused by the long wheelbase and lack of bogies.

Accordingly, each car was refitted with a more powerful Cummins L10 series engine – 230 bhp (170 kW) per car, which equals 460 bhp (340 kW) per twin-car unit – and Voith T 211 r two-stage hydrokinetic transmission,[5] starting with a torque converter which switches to fluid coupling drive once the unit is up to 45 mph (72 km/h).

The unit retained its Regional Railways livery and ran three trips a day all summer from Abbotsford to New Westminster.

[25][26] A total of seventeen Class 142 units based at Newton Heath TMD (142041-049 and 142051-058) were refurbished for use on Merseyside Passenger Transport Executive's City Line on services around the Liverpool and Greater Manchester areas.

The refurbishment included dot-matrix route indicators and a new design of individual low-backed seating, and the trains were repainted into Merseyrail's livery.

All of its Class 142 Pacer fleet wore the turquoise and cream house colours of Arriva Trains Wales.

The Class 142s received minor refurbishments with retrimmed seats in new moquette, plus the installation of ceiling-mounted CCTV cameras.

These were loaned from Northern (where they had been stored), in part to cover for the refurbishment of FGW's Sprinter fleets but also to allow the Class 158s to be reformed as three-coach sets.

[38] In December 2019, three withdrawn Arriva Rail North units were transferred to Transport for Wales for spare parts.

However the UK Government's Transport Secretary at the time Geoff Hoon denied this claim saying, "I would not accept that any of that rolling stock is unsafe", and that they constantly upgrade them.

"[95] Northern Rail insisted that the trains were safe, and that they are "subject to strict regular safety and maintenance checks".