Think of leaves on rails as black ice on roads and you'll begin to understand the nature of the problem. We're not talking about piles of dead leaves, but a hard slippery layer that coats the rails and is very difficult to remove.
Briefly, this is what happens:
- leaves are swept onto the track by the slipstream of passing trains
- light rain falls
- train wheels crush the wet leaves at a pressure of over 30 tonnes per square inch
- this compacts and carbonizes the leaves, forming a hard, Teflon-like coating on the rails.
Therefore, trains have to operate at slower speeds to ensure safety and to reduce the potential for wheel slip and spin. This means that drivers have to brake earlier for stations and signals and move off again more slowly. Consequently, train services can be delayed. If a train can't move because its wheels can't grip the rails, often there is no alternative route, therefore following trains are delayed or have to be cancelled.
In addition to causing severe disruption to customers, the damage inflicted on train wheels during sliding and spinning on rails is considerable and means some trains have to be taken out of service for an expensive repair. The rails too can be damaged costing many thousands of pounds to repair each year.
Fallen leaves really can disrupt rail services - not just here in Britain, but all over Europe and North America. The scale of the leaf-fall problem and the cost of keeping services running smoothly is huge:
- a mature lineside tree has between 10,000 and 50,000 leaves
- thousands of tonnes of leaves fall onto railway lines each year
- there are 20,000 miles of track to keep clear in Britain
- the annual cost of repairing damage to trains and track from leaf fall is over £10 million
- lineside vegetation management costs over £5 million each year
- the cost of felling large trees is between £20,000 and £50,000 per mile.
It is impossible to predict exactly when the leaf fall season will start and how long it will last, but the weather can provide a guide to its likely onset and how serious it is likely to be for the railway.
A severe leaf fall season often follows a wet summer. It starts with a hard frost, followed by a high wind and a period of dry weather, which causes large amounts of leaves to fall over a short period of time.
But traditionally, autumn is the season of mists and mellow weather, which spreads leaf fall over a longer period and reduces the severity of the problem.