Cheapest way to travel Europe by rail?

My boyfriend is going to study at Cardiff University in Wales for 6 weeks, and I get to go with him. We'll have some travel time, not sure how much yet - probably weekends plus maybe one full week. We want to see multiple countries (probably 6-12) during that time. We heard once that getting the rail pes in the USA would be cheaper. What pes should we get and where should we get them? We don't want to spend a fortune!