Toei Shinjuku Line through-running network

The Toei line opened in stages. The first section, Iwamotocho to Higashi-ojima, entered service on 21 December 1978. Through-running began with the next stage: on 16 March 1980 the Shinjuku–Iwamotocho section opened, and mutual through services started with Keio Teito Electric Railway — the company now known as Keio Corporation — over the Keio New Line, a relief line between Shinjuku and Sasazuka that had opened in October 1978. At first the subway tunnel east of Iwamotocho could only handle six-car trains, so the longer Keio sets turned back there; as platforms were lengthened, the section open to through Keio trains was pushed east to Ojima in December 1987 and to Moto-Yawata in September 1991. The Toei line itself reached its eastern terminus at Moto-Yawata, in Ichikawa, Chiba, on 19 March 1989, giving a finished route of 23.5 km.

Beyond Sasazuka the network fans out across the Keio system. Most trains continue from Shinjuku onto the Keio New Line; from there a majority run via the Keio Line and the Keio Sagamihara Line to Hashimoto and Keio-tama-center in the western suburbs, while the remainder turn back at Sasazuka. Through-running onto the Keio Dobutsuen Line began on 1 September 2006, though that service has been suspended and reinstated several times over the years. The reach extends even further west in limited workings: some peak-direction and weekend/holiday trains run beyond the Sagamihara Line junction onto the Keio Takao Line as far as Takaosanguchi, and historically an express linked Keio-hachioji on the Keio Line with Ojima. These longer-distance workings have always run eastbound into central Tokyo in the mornings and outbound in the evenings, matching commuter flows.

Matching train lengths was a recurring theme. Keio cars first ran in ten-car formations on the line from 1 September 1981, and from 1 September 2006 most Hashimoto-bound through trains were ten cars; eight-car Keio sets were withdrawn from the through services on 3 September 2007. Toei's own fleet followed, and from 11 August 2022 every train on the line runs as a ten-car formation. The shared 1,372 mm gauge remains the defining feature that makes this cross-operator network physically possible, and the line is the only Toei subway route to run outside Tokyo, crossing into Chiba at its eastern end while reaching deep into Kanagawa-bound Keio territory at its western end.

Most trains through-run from Shinjuku onto the Keio New Line. From there a majority continue via the Keio Line and Keio Sagamihara Line to Hashimoto and Keio-tama-center, while the rest turn back at Sasazuka; express services cover the full Shinjuku–Moto-Yawata trunk. Longer westbound reaches onto the Keio Takao Line (Takaosanguchi) and the Keio Dobutsuen Line have run at various times, typically as peak-direction or weekend/holiday workings rather than all-day service.

• 1978On 21 December 1978 the Toei Shinjuku Line opened its first section, between Iwamotocho and Higashi-ojima (6.8 km). The line had been planned as Line 10 and was given the name Shinjuku Line earlier that year.
• 1980On 16 March 1980 the Shinjuku–Iwamotocho section (7.3 km) opened and mutual through-running with Keio Teito Electric Railway (now Keio Corporation) began, routed onto the Keio New Line, which had entered service on 31 October 1978. Because the track east of Iwamotocho could initially accommodate only six-car trains, the longer eight- and ten-car Keio trains terminated at Iwamotocho.
• 1981On 1 September 1981 some Keio trains began operating into the line in ten-car formations.
• 1987On 20 December 1987 platforms were lengthened across the line and the section accepting through Keio trains was extended eastward from Iwamotocho to Ojima.
• 1989On 19 March 1989 the final section, between Shinozaki and Moto-Yawata, opened, completing the 23.5 km line between Shinjuku and Moto-Yawata in Ichikawa, Chiba.
• 1991On 1 September 1991 the permanent Moto-Yawata station opened and the section accepting through Keio trains was extended all the way to Moto-Yawata.
• 2006On 1 September 2006 most through trains bound for Hashimoto and Keio-tama-center via the Keio Sagamihara Line were lengthened to ten cars, and through-running onto the Keio Dobutsuen Line (reached via the Keio Line) began.
• 2007On 3 September 2007 eight-car Keio formations were withdrawn from the through services, which were standardised on ten-car trains.
• 2022On 11 August 2022 the line, long operated mainly with eight-car trains, switched entirely to ten-car operation across all services.