For engineers trains, the London Underground needed locomotives which could also operate when the third rail was switched off. Starting in 1905, these were created by mounting batteries in the passenger compartment of motor coaches. These could run under 630 V power from the third rail and switch to battery power when they reached the construction site. In 1936, a new type of purpose-built battery locomotives was introduced.

They re-used the bogies of old motor coaches together with their traction motors. They were built to the small Tube loading gauge and had cabs on both ends. When running on the batteries at 360 V, they produced considerably less power. By 1985, 52 locomotives had been built in six batches by four manufacturers. Although each batch had some improvements, they all used the same control equipment to enable multiple working. Only the last batch could recharge the batteries from the third rail, all others had to be recharged in the depot.