Although there are many subways which are running overground, usually these subway trains run above overground outside the downtown. Tokyo Metro's Marunouchi line is a unique line in Japan which runs overground several times in the city center. The trains run overground between Myogadani and Korakuen stations, between Ochanomizu and Awajicho stations and at Yotsuya station.
Marunouchi line is the second oldest subway in Tokyo, the fourth oldest in Japan after Ginza line in Tokyo, Midosuji and Yotsubashi lines in Osaka. Only Ginza and Marunouchi lines are standard gauge by Tokyo Metro and these two are also only subway lines in Tokyo which are using third rail method to provide electricity.
Marunouchi line is also one of the two Tokyo metro lines which does not operate through trains with other railway operators. The main reason is that all major private railway operators and JR East do not operate train with third rail method.
Marunouchi line is connecting Ikebukuro and Ogikubo, then it is only subway which runs into Tokyo station. Then, most of the subways run under roads but Marunouchi line are running under private lands. Also, Marunouchi line has a feeder line between Nakano-Sakaue and Honancho stations.