This class represents the first production batch of 1972 design stock and was based on the PEP prototype train sets used on the Southern Region. The 64 Class 313s were introduced for the electrification of the Kings Cross/Moorgate suburban system and emerged in 1976-77, being allocated to a new depot at Hornsey.
The sets were of novel design when introduced as they could operate from both ac and dc power supply from either the overhead connection at 25kV ac or a third rail in the tunnel sections at 750V dc
The sets were gangwayed within with end emergency doors for the tunnel sections. Passenger accommodation was in all open saloon style with passenger access by two pairs of bi-parting sliding doors on each vehicle.
The set remained deployed just on the Kings Cross route until modernisation of the Euston-Watford and Richmond-North Woolwich lines took place in May 1985, from when a batch of sets were transferred to Bletchley for this use.
When first introduced units were painted in blue/grey colours, this gave way to NSE livery after the introduction of the BR business sectors in the late 1980s.
Upon privatisation the sets remained on the same routes, being operated by Silverlink Railways on the Euston/Richmond lines and by WAGN on the Moorgate/GN routes. New liveries soon started to appear with Silverlink launching a rather gaudy colour scheme (see illustrations) and WAGN eventually applying its identity to Hornsey sets.
The 34 units operated by Silverlink started to receive a modernisation package in 1997, which slightly reduced the total seating but gave passengers an improved internal atmosphere with high back seats. Security measures were also incorporated including CCTV equipment.