Station on the Northern Line (Bank Branch), and Bakerloo Line opened on 18 December 1890. On the boundary between Travelcard Zone 1 and Travelcard Zone 2.
The station was built in two stages. The Northern Line station opened on 18 December 1890 as part of the first deep-level tube, the City & South London Railway (C&SLR). The Baker Street & Waterloo Railway (BS&WR) station opened on 5 August 1906, five months after the rest of the line. Although belonging to separate companies, the platforms were connected below ground from 10 August 1906.
There have been several proposals for a Bakerloo Line extension from the station.
See also Elephant & Castle (National Rail) Station
More information on the Wikipedia page [1]. The TfL page is [2].