Because of the various railway operators there is a selection of discounted fares for over 55s which vary from network to network. Some of the offrs are excellent for example it was possible to travel from London to Penzance, Cornwall in March 2009 for £25 return which is a huge saving over the normal price. And this is without having to purchase any travel card first.
If you Google 'cheap rail travel over 55s' it will bring up a list of operators and deals on offer. Proof of age is required when travelling.