Keisei Oshiage Line

The line dates to the very beginning of the Keisei company. On 3 November 1912 Keisei Electric Tramway (then 京成電気軌道) opened its first section, running from Oshiage through Magarikane (renamed Takasago in 1913, today Keisei-Takasago) to Iyoda (today Edogawa), together with part of what is now the Kanamachi Line. Oshiage was thus Keisei's original Tokyo terminal, and the Oshiage–Aoto stretch formed part of the company's main line. The line was laid to 1,372 mm gauge and at first included street-running track through the densely built shitamachi districts it still passes through today.

The early route was reshaped by public works and steady expansion. On 5 June 1922, to make way for the excavation of the Arakawa floodway, the section around present-day Yahiro and Tateishi was switched from shared street running onto a new dedicated alignment, eliminating the last street-running track on the entire Keisei system. New stations followed: Arakawa Station (today Yahiro) opened on 11 July 1923 and Aoto Station on 1 November 1928, while in November 1931 Hikifune and Tateishi stations were renamed Keisei-Hikifune and Keisei-Tateishi.

The opening of a more direct route into Tokyo redefined the line's role. Keisei had originally intended to extend from Oshiage toward Asakusa, but changed its plans, and once a line running from Ueno-kōen (today Keisei-Ueno) via Nippori to Aoto came into service in 1932, that route took over as the main line. In 1944 the section from Ueno-kōen to Keisei-Narita was formally designated the Main Line and the Oshiage–Aoto section the Oshiage Line. On 20 February 1945 the whole line was converted from a tramway under the Tramways Act to a railway under the Local Railway Act.

The most consequential changes came around 1960. On 11 November 1959 the entire line was regauged from 1,372 mm to 1,435 mm standard gauge, and on 4 December 1960 through services began with the Tokyo Metropolitan Bureau of Transportation's Line 1 (now the Toei Asakusa Line). This was the first through-running between a private railway and a subway in Japan, and it restored the Oshiage Line to its role as Keisei's connection to central Tokyo: thereafter it carried more passengers than the Keisei-Ueno and Nippori main-line terminals, providing trains from the main line into downtown Tokyo via the Toei subway.

Later decades wove the line ever more deeply into the regional network. After the Hokusō Line reached Keisei-Takasago in 1991, the Oshiage Line became part of the route between the Hokusō corridor and central Tokyo, and onward through-running to the Keikyu lines made it part of a chain linking Narita and Haneda airports on a single train. When the Narita Sky Access Line opened on 17 July 2010, a new Access Express service was introduced over the Oshiage Line and the older Express (kyūkō) service was discontinued; the same period also ended the line's through-running with the Kanamachi Line. The line thereby gained a second role, as a relay route handling access to both airports alongside its function as a connector to the Keisei Main Line.

In recent years the line has been progressively grade-separated to remove level crossings: the Oshiage–Yahiro section, including Keisei-Hikifune Station, was fully elevated by 22 August 2015, when the line's train-protection system was switched to C-ATS, and further elevation work between Yotsugi and Aoto remains planned. Today the Oshiage Line functions as a de facto main line of Keisei, busy enough that in fiscal 2024 its most crowded segment, Keisei-Hikifune to Oshiage, ran at a peak congestion rate of 138 percent. Keisei's flagship Skyliner, however, does not use the Oshiage Line, instead running to and from the main-line terminus at Keisei-Ueno.

• 19123 November: Keisei Electric Tramway opens its first section, Oshiage–Magarikane (today Keisei-Takasago)–Iyoda (today Edogawa), at 1,372 mm gauge; Oshiage is Keisei's original Tokyo terminus.
• 19225 June: to allow excavation of the Arakawa floodway, the Yahiro–Tateishi area is moved from street running onto a new dedicated line, ending the last street-running track on the Keisei system.
• 192311 July: Arakawa Station (today Yahiro) opens.
• 19281 November: Aoto Station opens.
• 1932The Ueno-kōen (today Keisei-Ueno)–Nippori–Aoto route comes into service and takes over as the main line; on 1 September Keisei-Ukeji Station opens between Oshiage and Keisei-Hikifune.
• 1944Ueno-kōen–Keisei-Narita is designated the Main Line and Oshiage–Aoto the Oshiage Line.
• 194520 February: the whole line is converted from a tramway (Tramways Act) to a railway (Local Railway Act).
• 19471 March: the suspended Keisei-Ukeji and Mukōjima stations are formally abolished.
• 195911 November: the entire line is regauged from 1,372 mm to 1,435 mm standard gauge.
• 19604 December: through services begin with Toei Subway Line 1 (now the Toei Asakusa Line) — the first through-running between a private railway and a subway in Japan.
• 19915 January: a tanker collides heavily with the old Arakawa bridge, bending the rails and suspending the Arakawa–Yotsugi section for several days; the Hokusō Line reaches Keisei-Takasago that year.
• 19941 April: Arakawa Station is renamed Yahiro.
• 201017 July: with the opening of the Narita Sky Access Line, the Access Express service is introduced over the line and the Express (kyūkō) service is discontinued; through-running with the Kanamachi Line had ended on 7 July.
• 201522 August: elevation of the Oshiage–Yahiro section (including Keisei-Hikifune Station) is completed and the line's train-protection system is switched to C-ATS.