Back in 2011, I created a pretty popular post (that now redirects here) that contained the WEP key for Softbank’s “mobilepoint” wifi hotspot on the Shinkansen. For some reason, both docomo and Softbank encrypt their public wifi connections with publicly accessible keys which doesn’t prevent eavesdropping on connections at all. Docomo even sells “visitor” access passes via their mobile portal, only accessible if you know the wifi password in the first place!
In any case, the passwords to get on the wifi on the Shinkansen or pretty much anywhere in Japan are now:
Once in, you can then log in with your roaming provider (I use Boingo, which works a treat in Japan, and is the source of the above info). The best “public” wifi provider in Japan is Wi2 if a hotspot is available – they provide a hotspot that doesn’t require a password and sells passes in English on the portal.