Using your own Car, The RAC now has an exelent Insurance service,

A Hire car can sometimes be the answer to a travel problem. The cost of a one way hire for four people would be less than the Train fare. Having used this myself when it was impossible to get Tickets for a journey from Madrid to the San Sebastian. In my case there were only two travelers but even then the overall cost was not outrageous. If you are willing to Drive you cant beat this mode of travel for convenience and as a bonus most hire car companies locate themselves close to Railway stations and Airports.

I can recommend Europcar. They seem to have offices just about everywhere in Europe.