With the electrification of the suburban lines of the LSWR, a large number of electric multiple units was needed. Since this would free lots of four-car permanently coupled coaches which had been built to be drawn by steam locomotives, the most economic way was to rebuilt these. So a number of these four-car units, built between 1902 to 1912 were used to create 84 electric three-car units.

Each end car had a cab and baggage compartment at the end and below these was a powered bogie with two 275 hp traction motors. Power could only be controlled via a simple on-off switch that automatically switched through the resistors until full power was reached. Because of the switching sounds of this machinery, the trains earned the nickname “nutcrackers”.

Over the years, the units got modifications like new, longer frames. Some units were withdrawn in the late thirties, but others were used as basis for the construction of the four-car 4-SUB sets starting in 1941. Also some unrebuilt units survived until the founding of British Railways and were withdrawn around 1950.