Tōkyū Tamagawa Line

The corridor opened on 11 March 1923, when the Meguro-Kamata Electric Railway (Meguro Kamata Dentetsu) — a company set up to develop the garden suburb of Den-en-chōfu — began running trains over the first section of its Meguro Line, between Meguro and Maruko (the present-day Numabe). This was the first line opened by the company that would become Tokyu, making the route the founding line of the entire Tokyu group. On 1 November 1923 the section from Maruko on to Kamata opened, completing the route, and the whole line was renamed the Mekama Line.

Through the 1920s and 1930s the southern part of the Mekama Line gradually filled in. Unoki Station opened on 29 February 1924, Shimomaruko on 2 May 1924, and a station called Honmonjimichi was added between Yaguchi (now Yaguchinowatashi) and Kamata on 12 October 1925. Several stations were also renamed in these years: Shinden became Musashi-nitta in 1924, Maruko became Musashi-maruko and then Numabe by 1926, the original Tamagawa Station was renamed Maruko-Tamagawa and later Tamagawaen-mae, Yaguchi became Yaguchinowatashi in 1930, and Honmonjimichi was renamed Dōzuka in 1936.

The Second World War brought the most dramatic change to the line's southern end. On 1 June 1945 the section running from Yaguchinowatashi through Dōzuka to Kamata was suspended, and it was formally abolished in 1946. In its place a new line between Yaguchinowatashi and Kamata opened on 14 August 1945, giving the route the alignment into Kamata that it still follows today and removing Dōzuka Station from the map.

In the post-war decades the line was upgraded as part of the Mekama Line. On 5 November 1955 the overhead voltage was raised from 600 V to 1,500 V. Stations continued to be renamed — Unoki took its present spelling in 1966 and Tamagawaen-mae became simply Tamagawaen in 1977 — and on 18 March 1989 trains were lengthened from three cars to four, though the parallel Ikegami Line stayed at three cars.

The line as it exists today was created in 2000, when Tokyu reorganised the Mekama Line. In preparation for separating the services at Tamagawaen, some trains were shortened back to three cars on 3 July 2000, and on 6 August 2000 the Tamagawa–Kamata section was split off from the Mekama Line and renamed the Tōkyū Tamagawa Line. On the same day Tamagawaen Station was renamed Tamagawa, one-person (wanman) driver-only operation began, and the line's rolling stock was reduced from four cars to three to share equipment with the Ikegami Line.

Since becoming an independent line the Tamagawa Line has continued to be operated together with the Ikegami Line, with which it shares all of its rolling stock. Daytime service was improved on 10 June 2005, when the average interval between trains was shortened from seven and a half minutes to six. New 7000 series trains entered service on the line on 9 January 2008. Tokyu has also studied extending the line roughly 800 metres east from Kamata to connect with the Keikyu network and improve access to Haneda Airport; as of June 2022, Ōta Ward had agreed to fund about 70 per cent of the project's cost, with the Tokyo Metropolitan Government covering the remaining 30 per cent.

• 192311 March: Meguro-Kamata Electric Railway opens the first section of its Meguro Line, Meguro–Maruko (now Numabe) — the founding line of what became Tokyu.
• 19231 November: the Maruko–Kamata section opens, completing the route; the whole line is renamed the Mekama Line.
• 192429 February: Unoki Station opens; Shimomaruko Station follows on 2 May.
• 192512 October: Honmonjimichi Station opens between Yaguchi (now Yaguchinowatashi) and Kamata.
• 193021 May: Yaguchi Station is renamed Yaguchinowatashi.
• 19361 January: Honmonjimichi Station is renamed Dōzuka.
• 19451 June: the Yaguchinowatashi–Dōzuka–Kamata section is suspended (abolished in 1946); on 14 August a new Yaguchinowatashi–Kamata line opens, fixing the present alignment into Kamata.
• 19555 November: the overhead voltage is raised from 600 V to 1,500 V.
• 197716 December: Tamagawaen-mae Station is renamed Tamagawaen.
• 198918 March: trains on the (then Mekama) line are lengthened from three cars to four; the Ikegami Line stays at three cars.
• 20006 August: the Tamagawa–Kamata section is split from the Mekama Line and renamed the Tōkyū Tamagawa Line; Tamagawaen Station is renamed Tamagawa, wanman driver-only operation begins, and trains are shortened to three cars to share stock with the Ikegami Line.
• 200510 June: the average daytime interval between trains is shortened from 7 minutes 30 seconds to 6 minutes.
• 20089 January: 7000 series trains enter revenue service on the line.