It’s that time of the year again when the days are getting shorter, it’s getting colder and the train service is impacted by falling leaves.
So why does something apparently as minor as a falling leaf cause such a problem to the modern railway, in an age when we can apply technological solutions to most problems?
Trains with steel wheels running on steel rails offer little rolling resistance to motion, reducing considerably the amount of energy required to move a heavy load. The contact point between the wheel and rail is approximately the size of a five pence coin and all the traction and braking forces of a train are transmitted through this small area. Compare that with the area and the friction between a car tyre and road surface. This normally works in the railway's favour, making it an energy-efficient mode of transport.
In autumn the advantage enjoyed by the railway becomes its Achilles heel as low adhesion caused by leaf fall impacts on our ability to brake and accelerate trains at the same rate as during the rest of the year.
Circular air currents generated by the front of a moving train pick up leaves and drop them onto the railhead in front of the train wheels. The wheels then crush these leaves at very high pressure onto the top of the rail. Add in a little moisture, dew or light rain, and the leaf debris forms a Teflon-like coating on the rail that reduces the adhesion between wheel and rail, causing the wheels to slip or, in extreme cases, lock up and slide.
Leaf contamination on a railhead
Typically Northern suffer an average 5% drop in our Public Performance Measure (PPM) during the autumn period. This is the performance measure we have to achieve to meet our franchise agreement and is measured in terms of the total number of our trains arriving at their final destination within 5 minutes of the advertised time. A 5% drop in autumn performance equates to an extra 131 trains failing PPM every day.
In addition when a train slides along the rail it causes “flats” to develop in the wheels. If these flats exceed a certain tolerance the train has to be taken out of service whilst the “flats” are removed. The resulting shortfall means that trains are sometimes shorter than usual, causing overcrowding, or even cancelled.
To counteract the impact of leaf fall causing contaminated rails, Network Rail will be again running their railhead treatment trains throughout the autumn period. These special trains spray a high pressure jet of water onto the railhead to clean leaf contamination from the top of the rail. The water is sprayed at such a high pressure that if the waterjets are left on when the train is stationary they will cut through the rail. They also deposit a mixture of sand and gel to the top of the rail, to enhance adhesion for following trains.
Network Rail’s treatment trains and the water jetting apparatus in action
Network Rail are also carrying out vegetation clearance works from the railway lineside. In addition to problems with leave fall during autumn, trees on the lineside cause us other problems, they fall across the tracks in high winds, encroach into overhead electric lines causing them to trip out, obscure signals and damage passing trains.
At Northern we train our drivers to drive in a professional manner in these conditions by braking earlier and lighter, and taking account of the areas where adhesion is likely to be worse - open areas exposed to sun and wind are likely to better than sheltered cuttings. All our trains are fitted with equipment which can deposit sand between the wheel and the rail to improve adhesion when the driver requires it.
On certain routes we operate autumn timetables between 3 October and 10 December 2016. These have increased journey times to help maintain punctuality during the autumn season. Details of these can be found at www.northernrailway.co.uk/travel/timetables.
Network Rail also have information at www.networkrail.co.uk/timetables-and-travel/delays-explained/autumn/ and www.networkrail.co.uk/community-relations/trees-and-plants/.