I can't recommend these more highly if you are in the US and traveling to your final destination by train. With my "4 days in 2 months pass," I can ride the Heathrow Express and on to Berwick, go to Edinburgh for the day, spend another in Glasgow, and then reverse to Heathrow for $239.
With the pass, it's cheaper and just as fast to take the train from London as it is to fly to Newcastle or Edinburgh and drive for an hour.
The trip I'll be making from Southampton to Berwick costs 81 GBP. I'll then have 3 days more to explore. The pass is good on any train at any time, except for certain trains at Christmastime. You show up and get on.
A few weeks ago, my sister and I went to York for the afternoon. We couldn't go into the Minster because there was a service going on. Perfect excuse to come back again. No, Little Shambles is the perfect excuse to come back.