The Japan Rail Pass (also commonly called JR Pass) is a very cost effective rail pass for long distance train travel in Japan. It can be used by foreign tourists only, and offers unlimited use of JR trains for one, two or three weeks at a cost that residents of Japan can only dream of. The pass comes in two types: ordinary and green car. The latter is valid on green cars (first class cars) that offer more spacious seats than ordinary cars. (http://www.japan-guide.com/e/e2361.html)
The official website for Japan Rail pass is here.
About the Japan Rail Pass and regional rail passes.
You can find free wifi spots in airports and coffee shops. You can also rent a pocket wifi (http://www.japan-guide.com/forum/quereadisplay.html?0+149380, http://www.japan-guide.com/forum/quereadisplay.html?0+124172).
Wifi in japan - japan-guide.com forum
Japan Question Forum: Wifi in japan.
Japan-Wifi.net pocket wifi reliable? - japan-guide.com forum
Japan Question Forum: Japan-Wifi.net pocket wifi reliable?.