The design was based on the front end of the Class 303 but with the gangway placed centrally and also incorporating a headcode box.

[7] The units were originally class AM9 (AC Multiple Unit) pre-TOPS, and were painted in the BR standard coaching stock maroon livery lined black & yellow, with the driving end gangway doors painted warning panel yellow.

The train would divide at Thorpe-le-Soken, with one of the 4-cars units used on the Walton section, and the remaining six cars continuing to Clacton.

In 1973-74 units 605–608 with the addition of TSK with TCK converted former Mk1 SK and CK coaches which formed in to 4 car sets.

In 1978 units 601–604 with the addition of TCK and TSOL converted former Mk1 CK and TSO coaches which formed in to 4 car sets.

The entire fleet was refurbished in the period 1985–1987, with the single-glazed wooden framed windows replaced by aluminium-framed double-glazed units with hopper ventilators.

The first refurbished units outshopped were painted in the bold new London and SouthEastern "Jaffa Cake" livery.

Coinciding with the refurbishment work, in 1985 electrification spread north from Colchester to Ipswich, and later to Harwich and Norwich.

309624, was repainted in a special blue livery to commemorate the opening a new railway line to Manchester International Airport in 1996.

However, the end was in sight, because as part of their franchise commitment, FNW had to replace their slam-door rolling stock, including the Class 309 units.

As a farewell gesture, three units were used on a final railtour from Manchester to their old haunt of Clacton-on-Sea via London Liverpool Street.

They were withdrawn in 2004, following completion of the tests and were stored at MoD Pig's Bay, near Shoeburyness, Essex, until early 2009 when they entered preservation.

However, once no further work was found for them in test train use, units 309616 and 309624 were successfully preserved by AMPSRail Limited in 2009, and went to the Electric Railway Museum, Warwickshire.

Minor conservation work was carried out, but due to changing priorities at the railway, the set was put up for sale in 2021.

BDTC 75965 and MBS 61928 have been donated to the East Anglian Railway Museum, where a full restoration of both vehicles has been committed to.

[18] Due to a lack of space at the EARM DTS 75972 has been donated to the Rushden, Higham and Wellingborough Railway who intend to restore it to operational condition.