In my fisking of Ross Clark I mentioned that I would explain the theory behind tilting trains. So here we go.

High-speed rail is simple, isn't it? All you need is more powerful locomotive and bingo... er, well, and, as with so many other things, not quite.

The trouble starts at 90mph. At that speed the noise from open windows is too great. So, you have to seal the windows which means that you have to provide some other means of keeping passengers cool in summer which means air-conditioning and, normally, a completely new fleet of rolling stock. That was one of the reasons for the introduction of the Mark II.

At 110-125mph you get another problem. Going round curves gets uncomfortable for passengers. (As I understand it, there is little danger of the train derailing. This doesn't happen until much later). Anyway, the discomfort for passengers is all to do with the g-forces. On a rather different scale this is precisely the problem that Formula One drivers face. The range of speeds I mentioned is related to the straightness of the track. In Britain the Western Main Line and the East Coast Main Line are relatively straight and we have had 125mph operation on each for over 20 years. The West Coast Main Line, on the other hand, is full of reverse curves which restricts its maximum speed to 110mph.

Er, hang about, that doesn't stack up. If the only problem is curves presumably you could do any speed you liked on a straight section. Hmm, dunno. But anyway, any old-ish railway is going to have plenty of curves and they will restrict the top speed of a conventional train.

So, what's the solution? The first solution, as pioneered by both the Japanese and the French is to build entirely new railways with the minimum number of curves possible making sure that what curves you do have are extremely gentle.

The second solution is to make the train tilt. That way the passenger experiences less lateral g-force, though, presumably, greater vertical g-force. I guess we're better at coping with one than the other. This is what we Brits tried but, ahem, failed to successfully introduce 20 years ago with the APT.

The problem with new lines is that they are extremely expensive. The problem with tilt is that you have to resite your signals. In Britain tilt also seems to mean carriages that are narrower at the top than at the bottom - so that they don't bang into one another on curves. Not good, especially when you bear in mind Britain's already restricted loading gauge.

There's one last thing. Above 150mph drivers can no longer see the signals so you need in-cab signalling.

Update 04/10/04 A reverse curve is a curve in one direction immediately followed by a curve in the opposite direction.

I should also point out that there is a third solution to the problem: don't bother. Something I generally favour.