The incorporation of the ELR took place on 26 May 1865 with the aim of providing a link between the LB&SCR, GER and SER lines.

[9] The companies reused the Thames Tunnel, built by Marc and Isambard Kingdom Brunel between 1825 and 1843 for horse-drawn carriages.

The company's engineer was Sir John Hawkshaw, who was also responsible for the major re-design and completion of I K Brunel's long-abandoned Clifton Suspension Bridge at Bristol.

The LB&SCR used its LBSCR A1 Class Terrier locomotives, which William Stroudley designed partly with this line in mind.

From March to September 1884 the SER service ran from Addiscombe to St Mary's (MR & MDR Joint Station).

Before the development of the Kent coalfields in the early part of the 20th century, house coal from the north for distribution in south London and as far afield as Maidstone and Brighton was an important source of revenue.

Access at the north end of the line was difficult: trains were limited to 26 wagons and had to be shunted into the Great Eastern's Liverpool Street station and drawn forward onto the ELR.

To avoid this reversal, a line was planned from the ELR north of Whitechapel to the GER at Bethnal Green.

Although the infrastructure was still privately owned, passenger services were operated as the "East London Branch" of the Metropolitan line.

The service to Hammersmith was reduced to peak hours only in 1936 and withdrawn in 1939,[14][15][16] leaving the East London branch as an isolated line.

[25] Maintenance passed to the Metronet consortium in 2003 under a public-private partnership, although the operation of trains continued to be the responsibility of Transport for London (TfL).

[26] The deepest point is at Wapping station, constructed in the Thames Tunnel's original entrance shaft 60 feet (18 m) below the surface.

A link with the Metropolitan and District lines was made just south of Whitechapel via St Mary's Curve.

Trains were operated by just a driver: the decision to withdraw the guards prompted an unsuccessful strike by the National Union of Railwaymen in May 1985.

[28] Unlike the previous East London line closure, no replacement bus service was provided across the River Thames through the Rotherhithe Tunnel.

As originally built the line had four stations, at Dalston Junction, Haggerston, Shoreditch and a large terminus at Broad Street.

At the start of the 20th century, more than one train per minute arrived or departed Broad Street during rush hour, with over 27 million passengers in 1902.

While Broad Street station's site was immediately sold for office use, becoming the Broadgate development, the route north was mothballed.

It was extended south to connect to the London Bridge arm of the Brighton Main Line, linked via a northbound flyover north of New Cross Gate.

Other than the new flyover and some associated works around New Cross Gate, it uses almost entirely existing track, running south to West Croydon via Brockley, Honor Oak Park, Forest Hill, Sydenham, Penge West, Crystal Palace (by way of a branch), Anerley and Norwood Junction.

[34] Transport for London acquired 20 new four-car Class 378 Bombardier Capitalstar electric multiple units to operate on the line.

The track and the northern extension remain under TfL ownership, and the stations from Dalston Junction to Surrey Quays are part of the London Overground network.

[35] The extension runs northwards from Whitechapel to Dalston Junction, and south to Crystal Palace and West Croydon.

The line was extended northwards to Highbury & Islington on 28 February 2011,[36] two months earlier than previously announced, with eight trains per hour during most of the day.

The first train, with headcode 9A20, was the 09:55 Highbury & Islington – Crystal Palace, which departed on time from platform 2 and was formed of a four-car class 378 unit.

[citation needed] A further 6.7 miles (10.8 km) link opened in 2012 from south of Surrey Quays via the Network Rail South London line to Clapham Junction via Queens Road Peckham, Peckham Rye, Denmark Hill, Clapham High Street and Wandsworth Road.

A new station at Surrey Canal Road was also planned, but this was put on hold in 2009,[37] though a suitable 'box' is being provided as part of the works to facilitate later implementation.

The planned lowering of Surrey Canal Road and associated work to the services were not carried out as a higher elevation was adopted, with a 1 in 30 (3.3 per cent) incline allowing the railway to cross at a suitable height.

[citation needed] To cope with increased demand, the class 378 units which operate on the line have been extended to five cars.