[3] Originally called the Baker Street & Waterloo Railway, the line was constructed by the Underground Electric Railways Company of London (UERL) and opened between Lambeth North (at the time named Kennington Road) and Baker Street on 10 March 1906.

[4] When work on the line started in June 1898, it had been financed by the mining entrepreneur and company promoter Whitaker Wright, who fell foul of the law over the financial proceedings involved and dramatically committed suicide at the Royal Courts of Justice, after being convicted in 1904.

As a result, work on the line was stopped for a few months and did not resume until Charles Yerkes and UERL stepped in and took over the project.

By the mid-1930s, the Metropolitan line was suffering from congestion caused by the limited capacity of its tracks between Baker Street and Finchley Road stations.

[12] Further extensions of the line were considered, south to Peckham Rye in the 1970s, and east to London Docklands and Canary Wharf in the 1980s.

The line celebrated its centenary on 10 March 2006, when events were organised with actors and staff in Edwardian costume entertaining travellers.

[18][19] In a former London Plan, it was projected that by 2026 the Bakerloo line would be re-extended from Harrow & Wealdstone to Watford Junction, restoring the pre-1982 service.

[18] The Best And Final Bid documentation for the Croxley Rail Link project indicates that this Bakerloo line extension is now "unlikely" because "TfL's plans to extend the Bakerloo line to Watford Junction are on hold indefinitely due to funding and business case constraints".

[20] Since the late 2000s, Transport for London (TfL) has been planning an extension of the line, with a route to Lewisham via Old Kent Road safeguarded in 2021.

[21] Four stations would be built, at Burgess Park, Old Kent Road, New Cross Gate and Lewisham,[22] with provision for a further extension along the Mid-Kent line to Hayes and Beckenham Junction.

[24] Due to TfL's poor finances following the COVID-19 pandemic, work to implement the extension is currently on hold.

A TfL Finance and Policy Committee Paper dated 11 March 2015 revealed that the repair programme for the 1972 Stock would be more expensive than anticipated, due to the unexpectedly inferior condition of the fleet.

[25] In early 2016, a four-year refurbishment programme began with the first of the new-look cars operating on the line in March.

[26] Each car's interior was cleaned, the seating moquette replaced with a variation of the Barman type seen on other lines, and handrails and lighting renewed.

According to a November 2021 paper by the TfL Finance Committee, replacement of the current trains may not occur until the late 2030s or early 2040s, due to a lack of funding.

[28] In the late 1990s, the Labour government initiated a public–private partnership (PPP) to reverse years of underinvestment in London Underground.

[32] In the mid 2010s, TfL began a process of ordering new rolling stock to replace trains on the Piccadilly, Central, Bakerloo and Waterloo & City lines.

[34] These trains would have an open gangway design, wider doorways, air conditioning and the ability to run automatically with a new signalling system.

[35] When opened in 1906, the Bakerloo line was operated by Gate Stock trains, built at Trafford Park, Manchester.

To cope with the extension to Queen's Park, 12 extra motor cars of the London Underground 1914 Stock were ordered, ten from Brush of Loughborough and two from the Leeds Forge Company.

To operate services north of Queen's Park, 72 additional cars were built by the Metropolitan Carriage, Waggon and Finance Company of Birmingham.

They were not equipped with air-operated doors and proved slow and unreliable, so they were replaced by new trains of Standard Stock by 1930 (although a few were retained by the LMS).