Holborn Viaduct
Holborn Viaduct is a road bridge in London and the name of the street which crosses it. It links Holborn, via Holborn Circus, with Newgate Street, in the City of London, England financial district, passing over Farringdon Street and the subterranean River Fleet. The viaduct spans the steep-sided Holborn Hill and the River Fleet valley at a length of 1,400 feet (430 m) and 80 feet (24 m) wide. City surveyor William Haywood was the architect and the engineer was Rowland Mason Ordish.

Holborn Viaduct is a road bridge in London and the name of the street which crosses it. It links Holborn, via Holborn Circus, with Newgate Street, in the City of London, England financial district, passing over Farringdon Street and the subterranean River Fleet. The viaduct spans the steep-sided Holborn Hill and the River Fleet valley at a length of 1,400 feet (430 m) and 80 feet (24 m) wide. City surveyor William Haywood was the architect and the engineer was Rowland Mason Ordish.
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