Has it become a reality?
If yes, what are the modalities for using it?
Thanks @pnuts for the link that contains the information I was looking for:
Step 1: Select the “Metro_Free_Wi-Fi” or “Toei_Subway_Free_Wi-Fi” SSID on your laptop or smart device.
Step 2: Launch your web browser.
Step 3: Follow the instructions on the screen (you will be required to register your email address), and you will be connected to the internet. You will be able to use the internet for up to 3 hours each time you log in.
By using the “Japan Connected free Wi-Fi” app and registering your details, you do not need to log into each Wi-Fi access point.
WiFi is not available in trains, only in stations.
Actually, only a selection of stations:
In addition to the answer by @nic, you can also use
JR-EAST FREE Wi-Fi at some JR stations, and
TOBU_Free_Wi-Fi at some Tobu line stations. This also requires you to register the service using email, for your information.
The official documentation is the following:
JR, available at Japanese, English, Korean, and Simplified and Traditional Chinese.
Tobu, available at Japanese, English, Korean, Thai, and Simplified and Traditional Chinese.
Also, you can see a list of available free Wi-Fi beyond the stations, including airport and bus station and some major cafe like Starbucks here.