Over 360 Class 158 diesel multiple units were originally built for Regional Railways, in the early 1990s at Derby, by ABB Transportation Ltd. Their high standards of performance, comfort and low running costs have made significant improvements to Express cross-country routes, where they subsequently displaced the remaining loco-hauled trains operated in the Provincial sector, as well as servicing some routes covered by earlier type Sprinters.
With a maximum speed of 90 miles per hour, the two-car consists are formed of a pair of Driving Motor Standard Class with Lavatory (DMSL) vehicles, one of which has a parcels secure storage area, designated DMSL(A), and the other a disabled persons toilet.
The bodyshells are fabricated from seam welded aluminium alloy extrusions, running on Series 4 bogies, with 70 seats in both cars - all cars are powered, the power train being an updated version of that used in the Class 150 Sprinter units. Passenger amenities are of InterCity standards, and includes air-conditioning, public telephones and provision for trolley catering services. Distinctive external features include the low air-dam front to the cab, and the use of dark tinted glazing in the cars side windows.