Built as replacement for the Class 306 and other suburban sets on the Liverpool Street-Shenfield route, these 61 4-car sets are a derivative of the 1972 high-density design. The sets are gangwayed within and have emergency end doors. The pantograph and transformer equipment is housed on one of the TS cars, while the driving cars have traction equipment.
The units were all delivered painted in standard BR blue/grey livery, which remained until the formation of Network SouthEast when this livery was applied. After privatisation when the sets became the property of First Great Eastern and emerged in their house colours.
When delivered these sets had a four-character route indicator display above the leading non-driving window, while a destination indicator was provided above the driving window.
Like the Class 314s, this series had dual sourced traction equipment's (from both GEC and Brush) as part of the 1980s policy of the rail industry not to be dependent on one supplier.