The early LBSCR AC overhead Electric multiple units (EMU) were phased-out by September 1929 and converted into DC types. All further electrification was at 660 V DC, and investment was made in modernising the fleet inherited from the pre-Grouping companies, and building new stock often by converting existing steam hauled carriages. The Southern Railway's EMU classification meant the unit type was given a three-letter code (sometimes two letters), prefixed by the number of carriages within each unit. These early suburban units, constructed between 1925 and 1937 were therefore designated 3-SUB, or later 4-SUB, depending on the number of coaches. The EMUs consisted of a fixed formation of two driving units at both ends of the train, and could have varying numbers of carriages in between (as indicated in the classification).