Network Rail
Network Rail Limited is the owner and infrastructure manager of most of the railway network in Great Britain. Network Rail is a non-departmental public body of the Department for Transport with no shareholders, which reinvests its income in the railways.

Network Rail Limited is the owner and infrastructure manager of most of the railway network in Great Britain. Network Rail is a non-departmental public body of the Department for Transport with no shareholders, which reinvests its income in the railways.
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