A total of 92 3-car sets were ordered just before Nationalisation in 1948 for the Liverpool Street-Shenfield line, these were delivered in 1949 and were very much of LNER design. Although built by Metro Cammell and Birmingham RCW, the sets had English Electric traction equipment. Originally the sets were fitted to operate at 1,500V dc from the overhead, this was collected by a cross-arm pantograph above the cab on the MBSO vehicle.
The two driving cabs had two forward facing windows, below on the non-driving side were four white lights, one located in each corner, with a central red tail light. The illumination of these white lights in sequence told the signalman the trains route. For passengers a roller blind route indicator was provided.
When it was agreed to convert the Liverpool Street electrification to the 25kV ac overhead system, the sets were rebuilt, much of the original equipment was retained, but a transformer and rectifier unit were added, as these could not be housed on the existing power car, they were slung between the bogies of the TSO, to facilitate easy power transfer, the pantograph and guards brake were also moved to the TSO, with the former guards space in the driving car being converted to passenger space. At the same time the white light route display system was replaced with a two character route indicator.
After conversion the AM6s, later Class 306s remained working on the Liverpool Street-Chelmsford route until displaced in the 1970s by Class 315s.
One Class 306 has been saved from disposal and is currently owned by First Great Eastern and kept for special traffic at Ilford depot.