Dollis Brook Viaduct
The Dollis Brook Viaduct, also known as the Dollis Road Viaduct, Dollis Viaduct or Mill Hill Viaduct, is a railway viaduct to the west of Finchley, North London, United Kingdom. It carries the London Underground's Northern line from Mill Hill East station to Finchley Central station. It is the highest point on the London Underground above ground level, reaching nearly 60 feet (18 m). It is located on a branch that was formerly part of the Edgware, Highgate and London Railway.

The Dollis Brook Viaduct, also known as the Dollis Road Viaduct, Dollis Viaduct or Mill Hill Viaduct, is a railway viaduct to the west of Finchley, North London, United Kingdom. It carries the London Underground's Northern line from Mill Hill East station to Finchley Central station. It is the highest point on the London Underground above ground level, reaching nearly 60 feet (18 m). It is located on a branch that was formerly part of the Edgware, Highgate and London Railway.
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