seishun 18 kippu (ticket) 青春１８きっぷ
Seishun 18 Ticket - Wikitravel
have a look at the periods for use.
i used these tickets numerous times when i was a student and i had a blast every time. it's like riding the underground or local train from osaka to tokyo. meet a person at maibara (a wee bit off kyoto) and she was going to hokkaido on those tickets. i wonder if she ever made it.