The British Rail Class 456 was an electric multiple unit passenger train introduced by Network SouthEast on inner-suburban services in South London to replace the elderly Class 416 2EPB units.^[3] Twenty-four two-car units were built by British Rail Engineering Limited's York Carriage Works in 1990 and 1991.^[1]

Following the privatisation of British Rail, the fleet was sold to Porterbrook and operated by Southern up until late 2013, when they were transferred to South West Trains and heavily refurbished.^[4][5] Routes that were operated by Class 456 units will be operated by new Class 701 Bombardier Aventra units in the future.^[6]

While originally approved for use by Network SouthEast (NSE) on services out of Waterloo,^[7] the 24 two-car units were first accepted into traffic as direct replacements for the Class 416 2EPB units on the Central Division of the Southern Region of British Rail.^[8] Units were delivered into traffic painted in NSE blue, red and white livery with cabs based on the Class 321 units, and were initially based at Selhurst depot.^[7] Although Network SouthEast had shifted to units painted with pale grey, the Class 456s were introduced painted with a darker grey shade to match the livery of the Class 455 units they would be working with. Units were numbered in the range 456001–456024, each unit consisting of a driving motor (DMSO) and a driving trailer (DTSO).^[8]

Entry into service for the Class 456 was originally planned for 18 March 1991, with trains to be driver-only operated (without the presence of a guard). This meant that the driver had to have a clear view of the platform with a bank of CCTV cameras mounted at the platform end for this purpose, however it was discovered that the position of the CCTV cameras on the platforms were not visible from the driver's position. To resolve this Selhurst depot designed a replacement fixing for the driver's seat so that it could slide sideways, allowing a good view of the CCTV units.^[9] The modified drivers' seats were installed at Fratton Depot with the work completed by the end of July 1991,^[7] and services with the new units commenced in September 1991.^[10]

In March 2005, No. 456006 was hauled to Wolverton where it was studied for corrosion assessment. This caused some doubt as to whether they would be refurbished.^[20] However, in a statement in April 2006 Southern said the class would receive a 'refresh', which meant they would not receive a refurbishment as major as the 455s, since the 456s are 10 years newer and already have high-backed seats (although of a different design). Also, new CCTV cameras and flooring were installed; this was not mentioned on the original refurbishment plan.^{[citation needed]}

All units were instead 'refreshed' and were painted into Southern green colours.^[20] No. 456006 was completed in a one-off promotional livery for rail safety on the Southern network. 456013 had an experimental cab-cooling system installed and 456022 had some experimental air-conditioning fitted, in an effort to make drivers' jobs more comfortable.

Southern removed the toilets from the trains, to provide more capacity. Despite this the Class 456 EMU trains lost two seats from each pair, due to a space being cleared for wheelchairs and pushchairs.

After the units transferred to South West Trains, a full refurbishment commenced in Wolverton where Class 456s received a new exterior livery and also a refurbished interior with new seating.^[21]

• On 15 August 2017, unit 456015 was damaged in a collision with an engineers train at Waterloo station, London.^[22][23] The cause was a wiring error in the signalling which meant that a set of points not correctly set was not detected. This was introduced as part of the testing of the temporary changes to the signalling during a major works programme at Waterloo. This allowed the signalling logic to detect that a set of points was correctly set when in reality, the points were mid-way between either of the correct positions. A false proceed signal was shown to the driver when it should not have been possible.^[23]