The British Rail Class 321 is a class of electric multiple unit (EMU) passenger train built by British Rail Engineering Limited's York Carriage Works in three batches between 1988 and 1991 for Network SouthEast and Regional Railways.

[2] In September 1987, Network SouthEast ordered 46 four-car units for use on services from London Liverpool Street to Cambridge and Southend Victoria, which became Class 321/3.

They also displaced many Class 312 slam-door units, which moved over to the London, Tilbury and Southend line.

The first of 30 321/3 units to be refurbished at Doncaster Works, as part of the Renatus project – which modernised the stock with features such as new air conditioning and heating, seating and Wi-Fi – was completed in December 2016.

These were built for outer-suburban services on the West Coast Main Line, from London Euston to Northampton, Rugby and Birmingham New Street.

[14] In 2007, 321408 and 321428 were loaned by Silverlink to c2c operating services out of Fenchurch Street while one Class 357 was repaired and another was undergoing tests in the Czech Republic.

The remaining seven (411–417) passed to Abellio ScotRail in 2015, after being converted to three-car Class 320/4 at Doncaster Works.

In June 1998 they were loaned to GNER to operate Leeds to London King's Cross services while its InterCity 225 fleet was grounded with mechanical issues.

[29] In November 2021 it was announced that 4 more Class 321s would be converted to Swift Express, with the first of these units expected to be delivered in February 2022.

[31] In September 2023, it was announced that Eversholt had abandoned the Swift Express project despite four Class 321 units having been converted.

To accommodate this sub-lease, three Class 321/3 units were transferred from National Express East Anglia to Silverlink.

[35] In May 2007, Central Trains began using Class 321 units on some morning services from Birmingham New Street to Walsall.

In November 2007, London Midland took over operation of the Class 321 fleet previously used by Silverlink and Central Trains.

[44][19][45][46] First Capital Connect received thirteen units (401–410, 418–420) from London Midland, following deliveries of the new Class 350/2s.

The units worked on the Great Northern services from London King's Cross to Peterborough and Cambridge from December 2010 until May 2017.

[47] Ten (402, 405–410 and 418–420) moved to Ilford depot for use by Greater Anglia,[48] with the remaining three (401, 403, 404) converted to Class 320/4s for Abellio ScotRail.

Having passed with the franchise to Arriva Rail North and Northern Trains, all were replaced by Class 331s with the last withdrawn in 2020.

The 321s were primarily used for services from London Liverpool Street to Braintree, Southend Victoria, Southminster, Ipswich (extending to Norwich during peak time), Walton-on-the-Naze, Clacton-on-Sea, and Colchester Town, but could operate services on any electrified route if other trains failed.

[54] Following a farewell tour on 29 April 2023, Greater Anglia completely withdrew its Class 321 fleet.

[60] In 2017 the prototype returned to Doncaster to be modified to the same specifications as the production units, including being refitted with hopper windows.

[63] In May 2018, plans were announced by Eversholt and Alstom to convert a number of Class 321 units to run on hydrogen fuel-cells, as a way of replacing diesel-powered trains on lines that are unlikely to be electrified.

[64] The details of the conversion project, dubbed 'Breeze', were published in January 2019, with an expectation that units could be ready for service in 2022 – although this later slipped to 2024.

[65][66] In November 2021 Alstom and Eversholt announced that they had decided to switch their primary focus to developing a hydrogen-fuelled variant of Alstom's Aventra EMU, on the grounds that the pool of convertible Class 321 units was too small to support the market they envisaged for hydrogen-powered trains in the UK, and that the Aventra's advanced technology and reduced requirement for routine maintenance were advantages in both manufacture and operations.

[67] Although the companies stated that they would maintain the "option" of converting existing trains in the future,[67] the Breeze project was ended in the first half of 2022 and at least one of the Class 321 units selected for conversion (321448) was sent for scrapping.