Removing those leaves on the line

Defining the problem

On the face of it, having a main line train delayed by a few fallen leaves seems preposterous. But it is actually more than a few leaves. And it isn’t so much the leaves themselves as what they do to the railhead that causes the difficulties. Steve Featherstone, Network Rail’s track director, explains: “As leaves fall onto the rails they are crushed by the wheels of passing trains. Over time more leaves fall and are crushed by more wheels of passing trains.

“This high pressure crushing turns them from things that we would recognise as leaves into the equivalent of a ‘Teflon’ coating on the rails. Teflon is more commonly found in non-stick frying pans and, whilst it is great for cooking, having non-stick rails is a major risk as the trains can just slide along the rails when they try to stop.”

In the autumn, leaves fall from trees all across the country. However, there are certain areas where the problem is known to be more significant. Adam Doy is the business manager (seasonal) at NDS.

“In terms of risk throughout the country, those areas where there is a lot of vegetation have the potential to suffer poorer adhesion and those areas where the network is exceptionally busy (London commuter zones) are always challenging. It tends to be the case that autumn starts in Scotland first and ends in the south in terms of leaf drop off. One significant variation depends on the type of trees which are adjacent to the railway with the larger leaves (sycamore) tending to fall first and the smaller (oak) later – again much of this is dependent on whether the summer has been wet or dry.”

The high pressure water jets run at 1,450 bar or 22,000 psi, and deliver a flow rate of 92 litres/ mile when running at 60mph (46 litres to each rail), so this is serious kit. The nozzles are designed by a specialist manufacturer (WOMA), which has the requisite skills and competence to deal with such a high pressure piece of equipment. They are normally used in industrial engineering metal cutting and have the ability to cut at a rate of 10mm/30 seconds. For safety, the system automatically shuts off when the train’s speed drops below 3mph to prevent any potential damage to the infrastructure.The adhesion modifier is delivered at a flow rate of 2-6 litres/minute, depending on the type of product being used and the speed of treatment (normally 40-60mph).

In addition to the RHTT, another six of which are currently on order with three being delivered this year, the same equipment is fitted to 32 multi-purpose vehicles (MPVs) so, at the peak of the leaf-fall season, 59 trains are deployed around the network.

Managing the operation

The operation of the water jet and adhesion modifier is controlled by an on board operator who has the ability to lay both treatment modes through on-board controls – Cattron remote control units (RHTT) and bespoke consoles (MPV). The operator, prior to each shift, is provided with a ‘tick sheet’ which denotes the exact locations where treatment is to be applied (water jet and/or adhesion modifier). These are known as ‘drop sites’.

The locations to be treated are compiled outside of the autumn season and are put together by an adhesion specialist within
the route team based on known areas of low adhesion, areas where there is a great deal of vegetation which has historically caused wheel slip, etc. In addition, Network Rail has the ability to alter treatment to respond to sites in the event of a station overrun or loss of train detection to improve adhesion and remove any ‘mulch’ caused by the wheels crushing the leaves.

Running any fleet of special trains on a crowded rail network is not without its complications. A certain amount of ‘down time’ is required back at one of the 22 autumn seasonal locations in order to replenish the water tanks, fill up the adhesion modifier and carry out the necessary examinations on the equipment. Getting this balance right and also getting the physical ‘paths’ on the network is always a challenging balance to obtain.

There are a number of operational restrictions depending on the location such as permissive working and platform sharing which have to be factored into the treatment circuits.

Network Rail also provides treatment on the Metropolitan line in order to service Chiltern Railways, so there is a need for close working with London Underground to ensure that the train plan is workable. Network Rail also needs to modify the locomotives so that they are ‘trip cock’ fitted in order to comply with LU rules and regulations as well as the FEA(F) wagons so that they are permitted to run.

Major exercise

Over the months of the leaf- fall season, the fleet of trains can cover a lot of ground. 3,500 treatment circuits are planned, making up one million miles of treatment at 230,000 individual sites. It’s a big operation and one which NDS plans for throughout the year.

And it is a problem that isn’t going away. In fact, various enhancements in other areas of train operation have made the leaf problem worse. Adam Doy explains: “There are a number of factors which have the ability to influence the situation. Modern lighter trains, which are more economical to run, are less aggressive to the infrastructure so they don’t clean the rails to the same degree as heavier trains. Changes in modern braking systems, from brake blocks to disc braking, no longer removes debris/mulch from the wheels. And alterations to the method in which the signalling systems operate (track circuit/ loss of train detection) make them more susceptible to failure through leaf fall than historic signalling systems.”

Cutting back vegetation so that trees are further from the track could alleviate matters. But the increasing amount of overhead electrification makes that problematic as much can only be done while the power is switched off, and in any case the cost of the vegetation control has to be factored into the equation.

Leaves mostly fall from mature trees, defined as trees that are 150mm in diameter at a height of 1 metre from the ground. There are 2.5 million mature trees within the Network Rail fence lines, not to mention the trees outside of the boundary that can also drop leaves onto the line. It costs about £1,000 to cut down and dispose of a mature lineside tree. So the total cost of clearing mature trees from the lineside would be about £2.5 billion, even assuming that the ecologists would let Network Rail do it.

So, as wholesale tree felling seems to be out of the equation, Network Rail has to deal with the ongoing problem at this time of year. With another three new trains to be delivered next year, the only solution seems to be to keep on blasting!