BY THE late 1930s, suburban third-rail electric multiple unit services into London’s southern termini were becoming increasingly overcrowded during peak times. Many examples of the flat-sided rolling stock, that dated as far back as 1914, had been converted from steam hauled coaching stock, catering for five passengers along each bench, or 10 per compartment.
One solution was to adopt curved sides, so a new generation of units with curved sides was created. Constructed from steel, sides were welded onto the frame, allowing passenger capacity to be increased by up to 20%. A prototype unit, numbered 4101, was built at Eastleigh, entering service for the Southern Railway in September 1941. It carried up to 456 passengers and was given the designation 4-SUB, indicating it was a four-car suburban unit. A second identical unit (4102), consisting of solely third class accommodation was constructed in 1944, with another eight units (4103-4110) produced in early 1945. These had distinctive dome-shaped roofs above the cab ends.
Eastleigh also manufactured 26 similar curved profile trailers. These were inserted into former London and South Western Railway (L&SWR) 1285 Series units, so they could be extended to four-car trains. A further 29 1496 Series units received similar trailer carriages in 1945-46. As a result, these units had an irregular profile, much like Class 455/7 units that have operated with inserted Class 508 non-driving trailers.
From 1946 onwards, a total of 184 new 4-SUB units were manufactured at Eastleigh. These differed, in that they had flat slab-like cab ends without the domed roofs. Production continued beyond Nationalisation, with breaks in numbering, until 1951. The last units entered passenger service in January 1952, being reclassified as
