[5] However, their introduction was delayed until 28 March 1983[6] due to an industrial dispute with the unions over driver-only operation,[7] and the diesel units lingered on until finally being replaced in mid-1983.

From 1987, new dual-voltage Class 319 units were introduced on the route, allowing the creation of a new cross-London service, from Bedford to Brighton, via Farringdon and City Thameslink station.

The Class 317 units were displaced to outer-suburban services on the WCML out of London Euston to Milton Keynes and Northampton.

The second batch units were built to operate outer-suburban trains on the Great Northern route from London King's Cross to Stevenage, Cambridge and Peterborough.

In 1986, the route came under the control of the newly created Network SouthEast, which introduced a new blue, red and white livery.

The extension of the overhead line equipment soon allowed the units to work services on the West Anglia route from London Liverpool Street to Cambridge.

In 1992, electrification spread north from Cambridge to Ely and King's Lynn, allowing the replacement of locomotive-hauled trains.

The closely related Class 318 units, which were built for the Ayrshire Coast electrification in Scotland, are effectively a three-car version of the same design, but with a lower speed capability of 90 mph (140 km/h), and with traction motors provided by Brush instead of GEC.

With the privatisation of Britain's railways, the Class 317 fleet was incorporated into the West Anglia Great Northern (WAGN) franchise.

Many of the WAGN Class 317/1 units still wore the Network SouthEast (NSE) blue and red livery dating from 1986.

Great Northern was temporarily operated independently under the WAGN brand name, until it was merged with the Thameslink franchise in April 2006.

These changes resulted in the Class 317 fleet being divided among First Capital Connect and National Express East Anglia.

First Capital Connect carried out a minor interior refresh to its 12 Class 317 EMU trains inherited from WAGN.

Many of the units hired to LTS Rail were repainted in a variation of the existing Network SouthEast blue, red and white livery.

In 2002, Thameslink began hiring four Class 317/1 units from WAGN to allow it to run additional Bedford to Moorgate services.

In 2004, when the WAGN franchise was split, twelve Class 317/1 units were transferred to Thameslink from the Great Northern route.

This was because a planned route blockade for engineering works meant that extra units were required for Bedford services.

They were restricted to services on the Northern half of the franchise only, as only dual-voltage or DC units can operate South of Farringdon .

The work was again carried out by Railcare at Wolverton, and included the fitting of luggage racks and new window frames, identical to existing 317/2 units.

Some Class 317/7 units had their original Stone Faiveley AMBR pantograph replaced by the Brecknell Willis High Speed design.

This changed when Abellio took over the East Anglia franchise on 5 February 2012, as the Class 317/7s were deemed too expensive to lease and surplus to requirements, and so they entered storage.

[18] The interiors were refurbished in a Metro style layout and provided a direct contrast to the 3+2 and 2+2 seating in use on the remainder of the unit.