Does pocket wifi work in the subway in Japan?
Let me go straight to the conclusion: Yes.
Even if the train is running underground, it works just fine.
FoxWifi uses only Softbank's cellular networks, which is one of the most stable and reliable networks you can get in Japan.
People spend time reading news, watching videos or reading books on their smartphones when they are on the train.
When can't you use it?
You have to be careful when you are going in a long tunnel in mountains on the way to suburban areas.
Even in shinkansen, sometimes it doesn't work in a tunnel, but usually, it shouldn't be a long time, so it's not a big deal.
If you are staying in big cities like Tokyo, Osaka, Yokohama, or Nagoya, it works in subways.
I've heard that you can't access the internet in subways in some countries, but no worries in Japan!