The Class 455 units were built in 3 variants between 1982 and 1985 at BREL York works. They are of all steel construction, with the exception of the TSO vehicles in Class 455/7 which have aluminium body shells, these vehicles being built in 1977 for Class 508 units. The body is based on the highly successful Mk3 coach design, adapted for suburban operation with the fitting of wider mid-vehicle doorways and constructed in a shorter length of 20 metres. The doorways originally had grab poles which restricted the through way to 1010mm, but Porterbrook is now modifying these to provide 1300mm through way, reducing station dwell times. The units have had extensive corrosion protection works and new interiors fitted offering a high degree of compliance with the RVAR. Electrical equipment is from GEC and is of a robust, low maintenance camshaft design. Couplers are Tightlock, with electrical connections made by jumpers on the cab front, allowing operation with Class 456 units. No air conditioning or toilets are provided.