Conventional wisdom for some years has been that, if a rail journey is less than three hours, it will attract more passengers than the parallel air service.

This is because most air journeys take at least three hours. You've got to get to the airport (say 30 minutes), check in and pass through security (an hour), wait (very few people cut it absolutely fine, so say up to 30 minutes), board, fly (at least 30 minutes), disembark and get to where you want to go (30 minutes).

If where you are coming from or where you want to go is at the airport - if you have a meeting there or are changing planes - then of course you can reduce the time: this changes the time equation and probably explains why people still fly between London and Paris or Brussels.

Equally, if the timetable doesn't match yours, you'll use the mode which suits you best - which is why there are two markets where rail carries more than air despite a 4.5 hour rail journey time.

I suspect that the 3 hour limit is growing. Why? Because rail travel time is much more useful time, much more usable time, than air travel time. Look at the calculation above - you are travelling for 3 hours but how much of that time can you actually use? Compare that with train travel time - once you are in your seat you are there, undisturbed, to do what you will. You can work (and many trains have lap-top connections): you can read, sleep, think, eat. You can use your phone (although it would be nice if you didn't share your conversation with the rest of the carriage!). You can talk to people. All of these are useful functions.

Indeed, I believe that in future calculations of generalised cost by different modes will need to account for the fact that while air travel time is a cost, with a negative value, rail travel time is a benefit, with positive value.

What do you think?