Tōzai Line through-running network

Through-running defined the line almost from the start. On 28 April 1966 trains began running through to the Japanese National Railways (JNR) Chūō Line as far as Ogikubo, initially as a one-way arrangement in which only Eidan trains crossed onto JNR track. Two-way mutual through-running followed on 1 October 1966, when JNR 301 series trains began entering the Tōzai Line. With the line fully open, the western through-service was extended to Mitaka on 8 April 1969, and on the same day through-running began at the eastern end onto the JNR Sōbu Line to Tsudanuma. The Sōbu connection settled into a rush-hour-focused pattern; from 2 October 1972 mutual through-running to Tsudanuma was set to run during peak periods only. When JNR was privatised on 1 April 1987, the Chūō and Sōbu through-running partner became the East Japan Railway Company (JR East); the relevant route is the Chūō–Sōbu Line (Local).

The network gained its second through-running partner in 1996. On 27 April 1996 the Tōyō Rapid Railway opened its Tōyō Rapid Line between Nishi-Funabashi and Tōyō-Katsutadai, and mutual through-running with the Tōzai Line began the same day. Functioning effectively as an eastward extension of the Tōzai Line, the Tōyō Rapid Line nevertheless remains a separate private operator to which the Tōzai Line offers through services. In 2004 the operator itself changed identity: on 1 April 2004 the Teito Rapid Transit Authority, commonly known as Eidan, was reorganised into Tokyo Metro Co., Ltd.

The result is a line with through services at both ends. At the western (Nakano) end, trains continue onto the Chūō–Sōbu Line toward Mitaka; at the eastern (Nishi-Funabashi) end, trains continue either onto the Chūō–Sōbu Line toward Tsudanuma or onto the Tōyō Rapid Line toward Tōyō-Katsutadai. The Tōzai Line is the busiest line on the Tokyo subway network, carrying a daily average of about 1.5 million passengers in 2024; figures cited by the Tokyo Metropolitan Bureau of Transportation in 2018 made it the most crowded train line in all of Japan, peaking at 199% of capacity between Kiba and Monzen-Nakachō. Its through-running reach into Chiba Prefecture, shared across three operators, makes it one of the most heavily used components of Tokyo's interconnected rail web.

The Tōzai Line through-runs at both ends. At the western end, trains continue onto the JR East Chūō–Sōbu Line (Local) to Mitaka, running all day except late at night. At the eastern end, trains continue onto the Tōyō Rapid Line to Tōyō-Katsutadai (all day) or, during weekday morning and evening rush only, onto the JR East Chūō–Sōbu Line (Local) to Tsudanuma. Outside the peaks, local trains through-run to the Chūō Line and rapid trains through-run to the Tōyō Rapid Line.

• 1964On 23 December 1964 the Tōzai Line opened its first section, Takadanobaba–Kudanshita (4.8 km), running 5000 series trains. The line had been formally named the Tōzai (East-West) Line on 27 October 1964, having been planned as Line 5.
• 1966On 28 April 1966 through-running with the Japanese National Railways (JNR) Chūō Line began, reaching Ogikubo. At first this was a one-way arrangement with only Eidan trains entering JNR territory; mutual (two-way) through-running began on 1 October 1966 when JNR 301 series trains entered the Tōzai Line. This was the first instance of a Tokyo subway line through-running with the national railway.
• 1969On 29 March 1969 the final section, Tōyōchō–Nishi-Funabashi (15.0 km), opened, completing the line. On 8 April 1969 the Chūō Line through-service was extended west to Mitaka, and through-running onto the JNR Sōbu Line to Tsudanuma began at the eastern end.
• 19728 April 1972: the all-day Sōbu Line through-service was curtailed to rush hours only (per the EN source, rev 1354376640). The JA source (rev 109677804) records 2 October 1972 as when rush-hour-only mutual through-running to Tsudanuma was formally established — likely two related events: the EN date marking the reduction in daily coverage, and the JA date marking a formal restructuring or regularisation of the remaining rush-hour-only pattern.
• 1987On 1 April 1987 JNR was privatised, and the Chūō and Sōbu line through-running partner became the East Japan Railway Company (JR East).
• 1996On 27 April 1996 the Tōyō Rapid Railway's Tōyō Rapid Line opened between Nishi-Funabashi and Tōyō-Katsutadai, and mutual through-running with the Tōzai Line began. The Tōyō Rapid Line functions effectively as an eastward extension of the Tōzai Line while remaining a separate private operator.
• 2004On 1 April 2004 the Teito Rapid Transit Authority (Eidan) was reorganised into Tokyo Metro Co., Ltd., which became the Tōzai Line operator.