From the Wikipedia page 
Transport for London Group Archives is the official historical business archive for Transport for London (TfL) and its predecessor bodies. It preserves and makes available to the public documents, photographs, plans and drawings relating to the company and its predecessor bodies, such as London Transport. It is based at TfL headquarters building, 55 Broadway, built over St James's Park underground station (District/Circle lines).
London Transport did not appoint an archivist until 1992, by which time a significant volume of records had accumulated in the vaults under 55 Broadway since the opening of the first Victorian underground lines in the 1860's. With the formation of Transport for London in 2000, the corporate archives became the responsibility of TfL Group Archives and Records Management department.
TfL Group Archives holds records from a large number of predecessor bodies, including:
- The London General Omnibus Company
- London County Council Tramways
- London United Tramways
- The Metropolitan Railway
- The District Railway - now the District Line
- The Central London Railway
- The City and South London Railway
- The Great Northern and City Railway
- The Hammersmith and City Railway
- The Underground Electric Railways Group
Some of the records of predecessor companies, such as the Metropolitan Railway Company and early minutes from the London Passenger Transport Board, are held on behalf of TfL by the London Metropolitan Archives.
The majority of TfL Group Archives holdings are records of:
- London Transport (in its various incarnations running 1933-2000)
- Transport for London (2000-present day)
Storage and Access
Documents are stored offsite in the work-out parts of Winsford Rock Salt Mine, Winsford, Cheshire, Britain's largest rock salt mine. The storage facility is operated by DeepStore Ltd, a records' management company. Access to the archives is restricted and is by advance appointment only. TfL Group Archives catalogue of all records held will be made available online and searchable to the public in 2008.