Sir: Dr Ian Craighead's letter of 25 February is misleading. Any competent car driver knows that adhesion limits certainly are affected by tyre pressure and the area in contact with the road. The third-form physics cited by Dr Craighead is inadequate because it assumes that the surfaces in contact are unaffected by the pressure, but in real life there are complex effects of localised distortion, melting and "stiction" (think of "dragster" or racing tyres) which are the subject of the branch of engineering known as tribology.
In the Seventies, I developed anti-block brakes for rail vehicles in Germany and I know the sledge brakes for trams that were referred to in previous correspondence. The magnetically operated version is basically a huge solenoid that can be driven by regenerative energy from the driving axles, producing a considerable magnetic clamping force. The wear on the brake sledge (which needs to be faced with a ceramic material similar to that used for cutting tools in lathes) and on the railhead is considerable, so they are only suitable for use at low speeds or in an emergency. There is a considerable scrubbing effect produced by these brakes, not by rubber- tyred vehicles that will not produce any scrubbing unless they themselves are slipping. German railways have used lightweight railbuses for a considerable time, and "leaves on the line" were a known hazard back in the days of steam traction.
If British Rail stopped using excuses and implemented technology that has been used for decades elsewhere, this topic would never have arisen. And if Dr Craighead thinks that adhesion limits are independent of surface area, then I hope for his sake that he never has to drive on ice.