Between 1975-78 a sizeable fleet of main line outer suburban 4-car sets were constructed at BREL York, based on the highly successful Class 310 fleet of the 1960s. This fleet were designated Class 312. The 49 sets built fell into three main groups 312/0 for Great Northern use on the newly electrified kings Cross routes, Class 312/1 for use on Great Eastern outer suburban lines which were fitted with both 6.25kV ac and 25 kV ac power collection and four sets classified as 312/2 for use in the West Midlands.
In specification and style the sets were almost identical to the 310s except for some minor redesign work on the front end.
When originally introduced the 312/1and 312/2 sets were finished in all over rail blue with full yellow ends, this gradually gave way to blue and grey, the 312/0 fleet were finished in blue-grey. Sets operated in the West Midlands sets were repainted in yellow/blue local colours.
After a short period of working in the West Midlands the four Class 312/2 sets were re-allocated to Hornsey and amalgamated with the Class 312/0 fleet. After the launch of NSE, all sets appeared in this colour scheme.
Following introduction of Class 317s on Great Northern routes the Class 312/0 fleet were re-allocated to Ilford for Great Eastern operation, while the 312/1 fleet later moved to the LTS line being allocated to East Ham, a handful of Class 312/0s were also transferred to the LTS area.
In the 1990s a number of upgrades to this slam door fleet were carried out, mainly by operator First Great Eastern who sealed up some of the passenger doors to try and reduce drafts and improve customer perception of the sets. After privatisation all FGE sets were repainted in the operators house colours.