Just to add to what Emma said, you can buy individual tickets for any train; you don't "need" to get a rail pass to ride trains.
If you post your itinerary and the trips you know you are taking as well as the trips you are thinking of taking, people can give you customized advice on whether a rail pass is a good deal for you.
In general these days, a rail pass can be a good deal for people taking a lot of long (therefore expensive) rail trips in a short time, or people who need or want full flexibility (last minute tickets can be very expensive on some routes). They are no longer a good deal for people who can buy tickets in advance for substantial discounts, and can live with the severe restrictions on refunds and exchanges these tickets have. And they were NEVER a good deal for people who just aren't taking that many trips.