In 1841 the LSWR opened a branch from Eastleigh (on the London-Southampton main line) to Gosport, with a ferry across the harbour to Portsmouth.

The people of Portsmouth and particularly the Admiralty thought such roundabout routes were unsatisfactory for a major naval base, especially when the rival port of Southampton, and Brighton, a mere seaside resort, were served by fast direct lines from London[citation needed].

It was operated and later acquired by the LSWR, and quickly became the most popular route to Portsmouth, despite initial obstructive tactics by the LBSCR at Havant.

[1] This put the LBSCR at a disadvantage, but over the following decade they developed a number of lines in Surrey and West Sussex that substantially improved their Portsmouth service.

It ran via Wallington, Sutton, Cheam and Ewell to an Epsom station on the east side of town.

[2] Twelve years later the independent Epsom and Leatherhead Railway (ELR) built a line from a new station on the west side of Epsom to Leatherhead, opening in February 1859 as a single line with just one intermediate station, at Ashtead.

The line connected to the ELR just east of the existing Leatherhead station, which became a terminus used by LSWR services only.

Although it primarily filled gaps in the LBSCR suburban network it also improved the route to Portsmouth, being slightly shorter than via Wallington,[note 7] and bypassed the bottlenecks at Norwood Junction and Selhurst; so it was promoted as a new "Portsmouth Line"[citation needed].

Contemporary maps label it the "South London & Sutton Junction Railway" suggesting that, as with other sections of the Portsmouth Line, this part may initially have been vested in an independent company to insulate the LBSCR from financial problems; although there is no evidence[verification needed] that they used such a vehicle for either construction or operation of the line.

On opening in 1868 the Sutton line had many links, all but one to other LBSCR lines: The section from Peckham Rye to Streatham passes through the grounds of Dulwich College and relatively hilly terrain, which required some costly engineering works: The arches of the viaduct and the south portal of Knight's Hill Tunnel are adorned with the monogram "AC", in tribute to the landowner Alleyn's College, as Dulwich College was known at the time the line was built.

This turned the old LSWR terminal at Leatherhead into a through station, and enabled a "Guildford via Epsom" service which continues to this day.

Also in 1927 the Southern Railway completely rebuilt Epsom LSWR station and layout to give the LBSCR (Sutton) line access to the platforms.

[5] The southern portal is framed by pilasters on each of which are two ball finials (stone spheres); it can be seen from Tulse Hill station.

It is on a curve and most unusual in that the southern portal is a skewed arch, with decorative details including massive stone capping, dentils and panelled brickwork.

It is 524 yards (479 m) long and runs through the lower chalk of Norbury Park, entering the hillside immediately north of one of the three viaducts over the River Mole.

[8] Restrictions imposed by the landowner, Thomas Grissell, meant that vertical ventilation shafts could not be constructed.

Corresponding to LOR SO680 is the Engineer's Line Reference (ELR) BTH ("Bermondsey to Horsham"), which is divided into 3 sections with mileage breaks at Sutton and Epsom.

After the Grouping the Southern Railway concluded that there was no longer a need for competing routes to Portsmouth and in 1924 withdrew the fast service from Victoria.