Train is definitely the best option for transportation. Whether or not a Eurail pass is the best way to pay for the trip is another question. If your itinerary is set (dates at least), then I’d look at buying point to point tickets from the individual national rail company websites. You should find advance purchase web specials for most of these routes. The links are here: http://www.eurotrip…. Use France or UK, not USA, as your country of residence on the French site to avoid being transferred to raileurope — where you’ll pay global fares (roughly 30% more). If you don’t find special fares, have problems actually purchasing the tickets (like your credit card is refused, or you can’t get an e-ticket or the specific train) or you don’t want to commit to exact trains several weeks in advance, then a 4 country select pass for 5 days (which, I see is what the trip planner recommends) will probably save money over buying full fare tickets at the station.

Have you looked into an open jaw flight? You might find a flight into Frankfurt and out of Paris or Amsterdam (or vice-versa) to be roughly the same as roundtrip to Frankfurt and you’ll save the time and expense of getting back to Frankfurt.