Class 306 trains were built to a pre-Second World War design by Birmingham Railway Carriage and Wagon Company (Driving Trailer) and Metro Cammell (Driving Motor Brake and Trailer) and were equipped with Metrovick traction equipment Crompton Parkinson traction motors.

[10] When built the trains were energised at 1,500 V direct current (DC) which was collected from overhead wires by a diamond pantograph located above the cab on the Motor Brake Second Open (MBSO) vehicle.

A transformer and rectifier unit was fitted to the underframe between the bogies of the intermediate Trailer Brake Second (TBS) and the pantograph, now a more modern Stone Faiveley AMBR design, was moved to the roof of this carriage.

[10] There is a record of a single three-coach unit hauling a Class 47 and train into Chelmsford after the locomotive failed on a London Liverpool Street-to-Norwich express.

Unit 306017 was preserved at Ilford depot; it had been repainted in a near-original green livery, albeit with a yellow warning panel on the front to comply with then-current safety regulations.