French and Spanish railways are considering a high speed service between Paris and Madrid, to start in 2012. Journey times will be between five and a half hours and six hours, depending on whether the trains run non-stop or call at Lyon and Barcelona.

Someone is assuming people will travel by a non-stop train for five and a half hours - fascinating.

In the past, it has been conventional wisdom that the break-even point for any city pair is three hours - over that journey time, more people will use plane than train. Three hours is about the minimum time it takes you to get anywhere by air - you have to get to the airport, go through security, board, taxi, fly, disembark, go through arrival procedures and then get to your final destination.

There are exceptions to the three hour rule. Paris - Perpignan and Tokyo - Hiroshima are the examples quoted where the rail journey is four and a half hours and yet more people use train than plane (although certainly in the case of Perpignan, a factor is the infrequent flights).

But, interestingly, in China the Beijing - Shanghai high speed line is being built - and there are plans for non-stop trains, taking 5 hours. So despite major improvements to Beijing Capital Airport and both of Shanghai's airports, it is still being assumed that there is a market for a 5-hour non-stop train. Fascinating.

Part of the reason may be the difference between the quality of the time on a train journey and on a plane journey.

If you decide to fly, your journey is very broken up - you never have much time anywhere, and when you do, you are restricted as to what you can do.

If you use a train, by contrast, your journey is pretty uninterrupted. You get on the train and the rest of the journey is yours - you can get in a lot of productive work!

So train travel time has a positive value: air travel time is a cost.