JR (Japan Railway Companies) has decided to invest in providing free WiFi on all the bullet trains (Shinkansen) by the end of 2019 due to strong demand by international visitors.
Free WiFi will be available on all the trains of Akita Shinkansen by the end of May 2019, Hokuriku Shinkansen by the end of September in 2019 and Tokaido, Sanyo, Kyushu and Yamagata Shinkansen will be ready by the end of 2019. Hokkaido, Tohoku and Joetsu Shinkansen will also increase this service gradually (no
schedule is provided so far though).
Currently, free WiFi service on Shinkansen is only available on some limited trains but it will be soon available on all of them with an eye on the Tokyo 2020 Olympics.
How to use Free WiFi and available service areas
News on NHK
Tokyo 2020 Olympics