Tokyo subway is huge, complicated, and overwhelming for a tourist. Now there is one good thing about it; it has free Wi-fi.
As part of the improvements leading to the 2020 Olympics, Tokyo Metro, the subway operator, has added Free wifi and apps that help calculate travel schedules.
That's quite a step forward. When I was in Tokyo a few years ago, very useful apps already existed and everybody was using them, but the apps were in Japanese and by Everybody I mean locals. And as a tourist you could easily get lost in this massive underground network.