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".

[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 ageing multiple units, particularly due to the necessity of handling and removing hazardous materials such as asbestos.

The first involved the development of substantial DMUs which could deliver performance superior to the existing fleet, especially on long-distance services.

Financial restraints led to the development of the second approach involving the so-called railbus that prioritised the minimisation of both initial (procurement) and ongoing (maintenance and operational) costs; 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 specifications or a high-density configuration.

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

[10] The Pacers had originally been intended as a low-cost stopgap solution to the rolling stock shortage, with a maximum lifespan of twenty years.

Outside of the UK, Pacer demonstrator units toured various nations, including the U.S., Northern Ireland, Belgium, Sweden, Thailand, Malaysia and Indonesia, without producing significant sales.

[12] The Pacer series was the result of an experiment to assess the viability of using bus parts to create a diesel multiple unit.

The two-car Class 140 was a prototype vehicle that was built in 1980 at British Rail Engineering Limited (BREL)'s Derby Litchurch Lane Works.

[13] 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.

The controller for the automatic transmission was observed by BR engineers to the cause of numerous operating failures and other reliability issues; this was allegedly due to defective relay logic and poor earthing.

They had a capacity of 94 passengers per two-car set, and two Leyland TL11 engines gave a total of 410 bhp (310 kW), resulting in a top speed of 75 miles per hour (121 km/h).

Furthermore, in order to maximise cost savings, the manufacturers made use of road bus-standard electrical equipment, passenger fittings, and general cab layout wherever possible.

[10] During 1988, in response to reliability problems related to the electronics used, the entire class was briefly withdrawn and underwent a technical upgrade at the Hunslet-Barclay works in Kilmarnock.

The reliability issue was largely resolved via the replacement of the original relay-based controller of the transmission with an alternative that was microprocessor-based.

The interiors were completely changed in 2000, when the Valley Lines service was introduced, with full back, coach-type seating installed throughout, along with improved fittings.

Instead of the usual bogies, Pacers use a basic four-wheel two-axle configuration which often results in a ride which is noisier and less comfortable than other trains.

The lack of articulation can result in a rough ride, especially over points, and a loud squealing noise around tight curves.

[7][12] Concerns were raised about safety after the 1999 Winsford crash,[20] which involved a First North Western Class 142 Pacer running as empty stock, which was involved in a collision with a Virgin Trains Class 87 express after the Pacer unit had fouled the main line at Winsford, Cheshire on the West Coast Main Line.

[22] Arguments have been made that Pacers were the saviour of some branch lines, with their low costs and relatively good fuel economy of 10 miles per imperial gallon (28 L/100 km; 8.3 mpg‑US)[12] making them a pragmatic solution at a time when budgets were tight.

Conversion of the old stock to mainline railway use will involve re-using the aluminium bodyshells, traction motors and bogies from the D78 units and fitting them out with new diesel engines and interiors.

The D-Train units underwent acceptance testing in 2015 and Vivarail pitched them to train operating companies (TOCs), especially those bidding for the Northern franchise.

[24] While no orders were received from Northern, Transport for Wales will replace its Pacers with a combination of five D-Trains and rolling stock cascaded from other train operators,[25] and other TOCs have ordered D-trains to replace other outdated rolling stock: South Western Railway for the Isle of Wight's Island Line,[26] and West Midlands Trains for the Marston Vale line.