Kingsland Viaduct
Kingsland Viaduct is a railway viaduct about 2 miles (3 km) in length from Shoreditch to Dalston, wholly within the present London Borough of Hackney in central and north-east part of London. It was built in the 1860s, but was disused from 1986 until it was reopened to carry the London Overground in 2010. The viaduct is owned by Transport for London. Since then it has carried East London Line services between Shoreditch High Street and Dalston.
Kingsland Viaduct is a railway viaduct about 2 miles (3 km) in length from Shoreditch to Dalston, wholly within the present London Borough of Hackney in central and north-east part of London. It was built in the 1860s, but was disused from 1986 until it was reopened to carry the London Overground in 2010. The viaduct is owned by Transport for London. Since then it has carried East London Line services between Shoreditch High Street and Dalston.
Check the official site before travelling; hours and prices can change.
More bridges

Tower Bridge
The bridge people mean when they say London Bridge. Walk the high-level walkway for the glass floor.

Millennium Bridge
Foot-only suspension bridge from St Paul's to Tate Modern — the only crossing aligned on St Paul's dome.

Hammersmith Bridge
Currently pedestrian-only, Victorian wrought-iron beauty — best photographed at golden hour.
Source: wiki-list