By 1907 the London and North Western Railway had got parliamentary approval to electrify their suburban services north-west of London and work had begun in 1909. At the same time they took over the management of the North London Railway and as a result of this the NLR route from Broad Street to Richmond was added to the system.

From the LSWR's point of view the last straw came in 1912 when a group of officials from Local Authorities from the South West of London made it known that they would support the Central London Railway in its proposal to also extend its line to Richmond and thence into the Thames valley and possibly in the direction of Chertsey. This resulted in the immediate announcement that the LSWR would electrify their innermost suburban lines from Waterloo.

The District and North London lines used systems where the outer conductor rail carried the 600 volts DC supply and the centre conductor rail was the return. The District was already running services over the LSWR's Putney to Wimbledon line and the decision was made that the LSWR should not copy the LBSCR, which had main line aspirations, but use the tried and tested system for suburban use. The side conductor rail with top contact by pick-up shoe would be the most suitable. The supply was to be 600 volts DC with negative return by way of the running rails. The only alteration that then needed to be made for the three systems to be compatible and services of the LSWR to run over the same track as the District and the NLR was for the centre conductor rail to be bonded to the running rails.

Eighty-four three-car electric multiple units were provided for this scheme. They set the trend for much of the electric stock built by the Southern Railway from then on in that they were rebuilt from existing ten-year-old steam hauled coaching stock. The accommodation was generally in compartment form but seating was a bit idiosyncratic resulting from the arrangement that four three car electric units were made up from three four-car steam hauled sets. Not only that but the original steam hauled sets catered for first, second and third class passengers whilst the new electric units only catered for first and third.

These units were promptly nicknamed "Nutcrackers" presumably because of the pronounced torpedo shaped driving end to the motor coaches. The cars were close coupled together within the unit and the electrical equipment was mounted in a small compartment behind the cab with a guard and luggage compartment at each end. The power bogies were mounted under the luggage compartment and had one 275hp motor for each axle giving a total of 1100hp for each three-car unit.