Most trains built since 1980 have disc brakes - where the brake is applied to a disc attached to the wheel - rather than block brakes applied directly to the wheel. The direct friction between the block and the wheel makes this type of brake more effective in removing leaf mould and other debris than disc brakes, although in normal circumstances disc brakes are superior.
New trains are also fitted with a wheel-slip protection (WSP) device, which means that its computer takes over braking when it detects slippage and it tries constantly to apply and release the brakes in an attempt to get adhesion. Drivers often dislike this device, because it means that when slippage is occurring, there is nothing they can do but hope the system works.
A report in Locomotive Journal, published by the drivers' union Aslef, suggests that in the Thames and Chiltern areas, "winter drizzle, mist or frost [was] causing excessive WSP. Confidence of driver undermined. Lengthy overruns show WSP a total failure." Tests by British Rail, however, have shown that WSP consistently outperforms drivers.
Rail managers are at a loss to explain why Turbo trains seem to have been particularly affected. There are only 80 of these diesel trains in use, all on the Thames and Chiltern lines, but they are of a very similar design to the electric-powered Networker trains that are widely used. Roger McDonald, the director of Thames, said that one possible reason may be that his "routes involve lots of stopping and starting, which means there is more opportunity for overruns".
Mr McDonald has expressed concern to Aslef about the incidents and says he is trying to improve safety for this autumn through better driver training, more tree removal and greater use of sandite trains, which are used overnight by Railtrack to spray an adhesive goo on the lines during the autumn.
He said that modifications including the reintroduction of sand on trains - many trains used to carry sand hoppers to cope with low adhesion - was being considered. There could either be a "one-shot" sanding unit, which would allow one application and is being tested this summer, or a more comprehensive version that would allow several applications. However, signal managers fear the widespread use of sand could interfere with lineside electronics.
A report by Aslef found that there was concern about the shortage of sandite trains in several areas including Birkenhead, Southend and Orpington.
Aslef is pressing for sand hoppers to be brought back. Keith Norman of Aslef said: "We want sand applicators to allow the drivers to put sand on the rail as often as necessary. The problem seems to be caused by the lightness of the trains and the use of disc brakes."
Terry Worrall, British Rail's director of safety, said that BR was tackling the problem. "We have taken a pro-active role in this which is why we commissioned the study and organised these conferences. There has always been a problem over rail adhesion, ever since the first trains ran on rails."
He accepted, however, that there were sometimes problems with the new brake systems. "There are certain rail-head conditions where disc brake units are more vulnerable than older brake systems." But he emphasised that in normal circumstances, disc brakes are much more effective than older types. As a result of its concern, British Rail has issued a new drivers' manual and a video about the new brake systems and has also issued new instructions to encourage drivers, signalmen and maintenance staff to report bad rail conditions.
Surprisingly, no single organisation appears to be collating full information on these incidents. Railtrack, the train operating companies, the Health and Safety Executive and British Rail all have some data but there is no statutory duty on any of them to record or act on the information. Next year, the HSE hopes that there will be new regulations ensuring that information about trains passing signals at red will have to be reported, but this will not apply to incidents involvingslides.