The Pacer series was the result of an experiment to see whether the possibility of using bus parts to create a diesel multiple unit was viable - the results of this are still undecided. The initial prototype, known as LEV-1, was a joint project by the British Rail Research Division and Leyland Motors using a bus body mounted on a modification of an existing freight vehicle underframe (HSFV1). This was followed by the two-car prototype class 140, which was built in 1984 at the British Rail Engineering Derby works.
The prototype was joined by another 20 two-car units which formed the Class 141 fleet. The units were used mainly in Yorkshire, operating on mainly suburban services. They had a capacity of 94 passengers per two-car set, and two Leyland TL11 engines gave a total of 410bhp, resulting in a top speed of 75 mph (121 km/h). The entire class underwent a technical upgrade in 1988 at the Hunslet-Barclay works in Kilmarnock. The units were withdrawn from use between 1997 & 1999. They have since been sold to the Islamic Republic of Iran Railways whilst a few remain in preservation. Because it used a standard Leyland National body, the Class 141 was narrower than the later Pacers, and could therefore accommodate only standard bus seating. The later Pacers had widened body panels to allow an increase in seating.
The next and largest Pacer class was the Class 142. This again was built by Leyland and BREL, in 1985. The body was based on a Leyland National bus, built at Lillyhall, Workington in Cumbria. Many fixtures and fittings of the Leyland National could be found on the train. The new class had a greater capacity of 106 passengers per two-car set and the same engines were used. The first sets were used initially on Devon and Cornwall branch lines and on commuter services in the Manchester area. The units from Cornwall were eventually moved to Manchester and the North-East, and the Class 142 has become a common sight on services across the North of England. The class was upgraded in the early 1990s to include more powerful engines, which gave a total power output of 460 bhp per two-car set. A number of trains were then modified for use on the Merseyside PTE city lines around Liverpool, which included dot-matrix route indicators, improved seating and Merseyrail PTE paintwork. This class moved into the control of First North Western at privatisation and subsequently passed on to Northern Rail and Arriva Trains Wales who have since operated it. Eight units have now been withdrawn from service, replaced by a cascading of British Rail Class 158s. First Great Western will receive up to 12 units to cover for refurbishment of their fleet and withdrawal of 12 Class 158 units for use by First ScotRail and East Midlands Trains. (Four 158s that were subleased from Northern Rail to First Great Western were also returned.)
Around the same time of the Class 142 development, a Pacer railbus was being developed by Kilmarnock-based Hunslet-Barclay. The train used a Walter Alexander bus body. The train was given the number Class 143 and entered service in 1985. Again with 2 205bhp motors giving a total output of 410 bhp and a top speed of 75 mph, the class originally had a capacity of 122 passengers. The class was used in the North East of England, before being transferred to Wales and was moved over to Wales & West control during privatisation. It then passed on to Wessex Trains, which became part of the Great Western franchise. The interior was completely changed in 2000, when the Valley Lines service was introduced, with full back, coach-type seating installed throughout, along with improved fittings. This reduced seating capacity to 106 seats per set. A similar Class 144 train, a Walter Alexander body on BREL underframe, was introduced in 1987. A unit was formed of either a two-car set with 122 seats or a three-car set with a total capacity of 195 passengers and 690 bhp, though still limited to 75 mph. The trains were used in the North East, passing to Northern Spirit at privatisation, then Arriva Trains Northern and now Northern Rail.
Doubts were raised about safety after the Winsford crash, which involved an empty First North Western Class 142 colliding with a Virgin Trains Class 87 and coaching stock at Winsford, Cheshire on the West Coast Main Line.
No railbuses have been produced in Britain since the Pacer classes.