The is a rapid transit line in Osaka, Japan, operated by Osaka Metro. Constructed under Midà Âsuji, a major north-south street, it is the oldest line in the Osaka subway system and the second oldest in Japan, following the Tokyo Metro Ginza Line. Its official name is , while the Osaka Municipal Transportation Bureau refers to it as , and in MLIT publications it is referred to as . On line maps, stations on the Midà Âsuji Line are indicated with the letter "M".
North of Nakatsu it runs above ground in the median of Shin-midà Âsuji, an elevated freeway.
The section between and is owned and operated by , but is seamless to the passengers except with respect to fare calculations.
In June 2018, the Midà Âsuji Line is the most congested railway line in the Kansai region of Japan, at its peak running at 151% capacity between Umeda and Yodoyabashi stations.
The Midà Âsuji Line was the first subway line in Osaka and the first government-operated subway line in Japan. Its construction was partly an effort to give work to the many unemployed people in Osaka during the early 1930s. The initial tunnel from Umeda to Shinsaibashi, as well as the Umeda depot, were constructed entirely by hand and opened in 1933 after being initially plagued by cave-ins and water leakage caused by the poor composition of the earth below northern Osaka and the equally poor engineering skills of the work crew. The first cars were hauled onto the line by manpower and pack animals from the Government Railway tracks near Umeda.
Although the line only operated with single cars at first, its stations were designed from the outset to handle trains of up to eight cars. The line was gradually extended over the next few decades, completing its current length in 1987, making it the second-longest subway line in Osaka after the Tanimachi Line (excluding the Kita-Osaka Kyà «kà  Railway extension of the Midà Âsuji Line).
All stations are located within Osaka Prefecture.
Fares vary according to travel distance, ranging from 190 to 390 yen for adults and from 100 to 200 yen for children. As the distance increases, the fare rises in stages. Passengers also have the added option of using IC e-cards.
Commuter passes come in the form of IC e-cards and can be purchased at Umeda station, Namba station, or Tennoji station at their respective sales counters or at the pink self-service machines in any station. Passes are available for 1-month, 3-month, and 6-month durations. Commuter pass prices vary depending on the duration and distance traveled in a designated route. Discounts are also available for students and children.
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Since 1987, all Midà Âsuji Line rolling stock operated by Osaka Metro are stored and maintained at Nakamozu Depot, the first underground depot in the Osaka Metro system. The first depot for the Midà Âsuji Line was located near Umeda station, which was replaced by Abeno Depot in 1950, Nagai Depot in 1954 (now used mainly for maintenance-of-way vehicles), and Abiko Depot in 1960 (closed in 1987). Since 2016, it is also possible for Midà Âsuji Line rolling stock to access Midorigi Depot on the Yotsubashi Line via pointworks before Daikokuchà  station, after heavier repairs and maintenance of all third-rail-powered Osaka Metro rolling stock were consolidated there.
Women-only cars were introduced on the line from 11 November 2002. There is one such designated car in each train (Car No. 6), the use of which is restricted all day on weekdays.