Baker Street station was opened by the MR on 10 January 1863 (these platforms are now served by the Circle and Hammersmith & City lines). On 13 April 1868, the MR opened the first section of Metropolitan and St John's Wood Railway as a branch from its existing route. This line, serving the open-air platforms, was steadily extended to Willesden Green and northwards, finally reaching Aylesbury Town and Verney Junction (some 50 miles/80 km from Baker Street) in 1892. The MR station mainly competed for traffic with Euston, where the LNWR provided local services to Middlesex and Watford, and later with Marylebone, where the GCR provided expresses to Aylesbury and beyond on the same line.
Over the next few decades this section of the station was extensively rebuilt to provide four platforms. The current Metropolitan line layout largely dates from 1925, and the bulk of the surface buildings, designed by architect Charles Clark, also date from this period.
The Baker Street & Waterloo Railway (BS&WR, now the Bakerloo line) opened on 10 March 1906; Baker Street was the temporary northern terminus of the line until it was extended to Marylebone station on 27 March 1907. The original station building stood on Baker Street and served the tube platforms with lifts, but these were supplemented with escalators in 1914, linking the Metropolitan line and the Bakerloo line platforms by a new concourse excavated under the Metropolitan line.
On 20 November 1939, following the construction of an additional southbound platform and connecting tube tunnels between Baker Street and Finchley Road stations, the Bakerloo line took over the Metropolitan line's stopping services between Finchley Road and Wembley Park and its Stanmore branch. The current Bakerloo ticket hall and escalators to the lower concourse were provided in conjunction with the new service. The Jubilee line added an extra northbound platform and replaced the Bakerloo line service to Stanmore from its opening on 1 May 1979.
On 23 August 1973, a bomb was found in a carrier bag in the ticket hall.[citation needed] The bomb was defused by the bomb squad. A week later, on 30 August, a member of staff found another bomb left on the overbridge. Again, it was defused without any injury.
Of the MR's original stations, the sub-surface Circle and Hammersmith and City line platforms are the best preserved. Plaques along the platform show old plans and photographs of the station.
The station layout is rather complex. The sub-surface station is connected to the open-air Metropolitan line station. This is a terminus for some Metropolitan line trains, but there is also a connecting curve that joins to the Circle line just beyond the platforms that allows Metropolitan line trains to run to Aldgate in the City of London.
Below this is a deep-level tube station for the Bakerloo and Jubilee lines. These are arranged in a cross-platform interchange, and there are connections between the two lines just to the north of the station. With ten platforms overall, Baker Street has the most London Underground platforms of any station on the network.
Outside the Marylebone Road exits, a large statue of Sherlock Holmes commemorates the fictional detective's association with 221B Baker Street. A restoration in the 1980s on the oldest portion of the Baker Street station brought it back to something similar to its 1863 appearance.