There were three batches of Class 455 units, all consisting of 4 cars: driving carriages at each end, an intermediate trailer vehicle and an intermediate motorised vehicle (powered by four GEC507-20J of 185 kW carried on the bogies of the MSO vehicle, some recovered from Class 405),all originally built to the Standard Class 3+2 seating arrangement. (Technically, they are formed DTSO+MSO+TSO+DTSO.) They have the same bodyshell as the Class 317 and Class 318, but as they were designed for inner suburban services they do not feature first class seating or toilet facilities and are restricted to 75 mph(121 km/h). Like the Class 317/318, as well as the diesel Class 150, they are based on British Rail's Mark 3 coaches, with a steel construction, unlike the earlier PEP-based Class 313s, 314s, 315