SR class 3Sub

Southern Railway 3-Sub Units
South Eastern division electric multiple units sandwich a trailer set. Orpington, 1928
In service	1915-48
Formation	power car + trailer car + power car
Operator(s)	London and South Western Railway, Southern Railway and British Railways (Southern Region).
Specifications
Maximum speed	75 mph 121 km/h
Prime mover(s)	Early units: 4 x 275 hp (205 kW) traction motors total 1,100 hp (820 kW) Later units: 4 x 250 hp (190 kW) traction motors total 1,000 hp (750 kW)
Braking system(s)	Air (Westinghouse)
Track gauge	4 ft 8 1⁄2 in (1,435 mm)

This article covers the DC suburban electric multiple units introduced by the London and South Western Railway in 1915 and those introduced by the Southern Railway in the period up to 1939. The class designation 3SUB was not used by the Southern Railway, although some authors refer to these units as 3SUB. When rebuilt to four cars in the 1940s, they became class 4SUB.

The London, Brighton and South Coast Railway (LBSC) was the first of the three major companies that were to form the Southern Railway to electrify some of its lines in London. The lines were electrified at 6.7kV 25Hz AC, using overhead supply. Public services began on 1 December 1909.

The London and South Western Railway (LSWR) electrified its lines on the third rail DC system. Public services began on 25 October 1915.

In 1922, the South Eastern and Chatham Railway (SECR) sought permission to electrify certain lines in London. On 1 January 1923, the LSBC, LSWR and SECR were merged into the Southern Railway (SR).

Eighty-four electric multiple units, numbered E1–E84, were constructed in 1913 by converting steam-hauled carriages which had been built from 1904 for suburban service. The conversions being carried out at Eastleigh Works. Each multiple unit comprised two motor coaches and a trailerd. The units were either 157 feet 5 inches (47.98 m) or 159 feet 5 inches (48.59 m) long, depending on whether a 49 feet (14.94 m) or 51 feet (15.54 m) trailer was included. The units weighed 95 tons. They seated between 172 and 190 people in first and third class. These were the first trains on the LSWR not to offer second class accommodation. Electrical equipment was by the British Westinghouse Company, each motor carriage have two 275 horsepower (205 kW) electric motors. In 1914, 24 two-coach trailer units were constructed at Eastleigh to work with the multiple units. Each comprised an eight-compartment and a nine-compartment carriage. The trailer sets were either 105 feet (32.00 m) or 107 feet (32.61 m) long and weighed 46 tons. They were third class only. The SR renumbered the electric multiple units 1201-84 and the trailer sets 1001-24.