Leaf fall on the London Underground

London Underground (LU). The clue is in the name – “underground”. So how does the organisation have a leaf-fall problem? Despite the “underground” in the name, LU has large lengths of route that lie out in the open rather than down in tunnels. On the Piccadilly line, there are such lengths in the east and west of the route, particularly the latter.

As a result, over the last couple of autumns, the Piccadilly line has had serious issues with poor wheel/rail adhesion. This problem peaked last year when the service frequency was affected by the number of trains that were out of commission as a result of wheel flats.

Spinning and sliding

When rails are slippery, the trains’ wheels slip when the train driver tries to accelerate, and slide under braking. The former can lead to ‘wheel burns’, damage to the rail head caused by the heat generated by the spinning wheels. When trains slide under braking, the wheels are damaged, potentially leaving a flat spot, and the rail may also be damaged.

Those who drive cars will probably have experienced skids, and possibly wheel spin, and may know that modern cars usually have systems to prevent both phenomena. These systems are pretty effective, so why do trains not have similar precautions built into them?

Modern rolling stock does, of course, but older trains, such as the Piccadilly’s 1973 stock, may not, and those that do may have systems that are not as effective as those of road vehicles. It is a lot trickier to deal with the issue when wheel and rail are both made of steel than it is when the tyre is rubber and has a tread pattern designed to remove water and other contaminants from the road surface.

Wheel flats are a problem for several reasons. They are noisy and cause vibration, and at best this is uncomfortable for passengers and irritating for neighbours. The vibrations cause damage to the trains, which may be significant in the case of bad flats. They also damage the track, and in extreme cases, may cause rail breaks. For these reasons, LU has to withdraw vehicles from service when wheel flats become sufficiently serious.

Damaged wheelsets may be repaired by turning them on a wheel lathe in one of LU’s depots, restoring them to the correct round shape. In more severe cases, though, or if they have been turned before and have lost too much metal to be turned again, wheelsets may need to be replaced with new ones.

There are two depots with wheel lathes on the Piccadilly line. However, during the leaf-fall season in 2016, the number of vehicles requiring attention exceeded the capacity of these to such a degree that it became necessary, on some days, to curtail train services on the line.

Plan for improvement

Something needed to be done, and before the next leaf-fall season in 2017. LU commissioned Xanta, a specialist rail consultancy, to examine the causes and recommend potential strategies to prevent recurrence. Xanta’s David Crawley, who has had many years of rail experience, carried out the necessary review and produced the report.

The recommended strategy entailed several strands of work. Managing lineside vegetation to minimise the leaf-fall impact on the line was one.

Working with train drivers to learn which they knew to be the worst problem locations was a second. This element also needed to include work to improve drivers’ appreciation of the problem and how to drive to minimise it.

Modifying the wheel/rail interface conditions to improve adhesion was a third strand. It was decided to modify the timetable during the leaf-fall season to ensure a reduction in the risk of slips and slides.

Lastly, to reinforce the work with drivers, signs were to be erected on the lineside at poor adhesion sites.

It was also recognised that all of these measures could only reduce the problem, and would not eliminate it entirely. It was therefore recommended that resources at the two maintenance train depots should be enhanced to enable them to cope more effectively with wheelset damage during the leaf-fall season. Additional staff were to be recruited and trained so that the depots could operate 24/7 train lifting facilities during the problem period. In addition, more spare wheelsets were to be procured, so that there would be a greater supply on hand should large numbers need changing in a short period of time.

Dave White, LU’s programme lead for the project, told Rail Engineer that all of these recommendations, which were in line with current best practice in the industry (at Network Rail and on the Metropolitan line for example), were accepted and have been implemented through a project with a £6.5 million budget. He described how this has been done in an interview with the magazine.

RATs on the line

The most obvious sign will be the two RATs – not scary rodents, but rail adhesion trains – that, from late September, will be running on the Piccadilly line’s above-ground sections. One, based at Cockfosters, will work on the eastern end of the line and the other, run from Northfields, on the western. Each carries 250 litres of adhesion modifier that is spread on the rail head in the areas that are at risk, working in a similar fashion to the Sandite trains employed on the main line network. The quantity of material carried is sufficient for the day’s work expected of each train.

The RATs are based on converted three-car 1973-stock passenger units that have a cab at each end (known as double-ended units), modified to carry the necessary equipment for the task. Dave said that the trains are completed and driver training is being undertaken, ready for the planned implementation date at the start of October. A further small modification is planned to make their brakes smoother in operation. Currently, step one of the braking of these older trains is a little abrupt, but there are two unused steps available in the braking system. The modification will bring these extra steps into use to “smooth” the braking curve, further reducing the risk of sliding.

Other adhesion management is employed too. There are TGAs, fixed “traction gel applicators”, in certain locations where adhesion has historically been a problem. Two additional TGAs have been procured and installed on the western end of the route, and the positions of the existing ones have been reviewed and optimised. This last has been done in collaboration with the drivers, who have been able to identify which TGAs that were not best positioned and advise how to improve matters.

Having mentioned the drivers, Dave went on to explain how the leaf-fall/adhesion issue will be incorporated into the regular annual driver competency refresher training sessions. Awareness of the problem will therefore be raised, and LU has also used feedback from drivers to assist the project.

The project has installed warning signs at locations where poor adhesion is likely, to remind drivers of the risk. The projects work with Piccadilly line drivers means that these signs will have real meaning for them, rather than potentially being ineffectual tokens.

The revised ‘leaf-fall’ timetable will see train speeds reduced from early October until mid December. Lower speeds reduce the risk of sliding under braking as deceleration rates can be lower, and also mean that acceleration rates need not be so great, which reduces the risk of wheel spin.

Vegetation control

LU has a standard for the management of lineside vegetation that was introduced, amongst other reasons, to control the risks from leaf-fall. However, Dave suggested that tree growth appears to have accelerated in recent years. This and, possibly some lack of priority from management, have left a problem needing action. The project has been addressing this seriously and is well through the task of removing lineside trees from places where they should not be.

Trees are now treated as assets, listed in an asset register and subjected to an asset management regime and a work programme. Laser scans have been used to identify trees within the designated clear zone adjacent to the line. The trees of neighbours are not forgotten, if they are of a species and in a location that means they are a potential source of problems. Every effort is made to obtain the cooperation of the owners in order to ensure that they are managed to minimise the leaf fall on the LU lines.

There have been comments from neighbours who have seen lineside trees disappear from their locality, and clearly not everyone welcomes such work, but the project has been active in warning neighbours and explaining why the action is essential. Dave was happy that the project team has been successful in managing this potentially tricky issue. This is quite an achievement for what has been described as the most intensive de-vegetation campaign LU has ever undertaken.

The final element in the project strategy is the use of weather predictions to permit the anticipation of poor adhesion by time and location. The project has been working to obtain the most accurate weather prediction information available and a contract is now in place with the Met Office. It enables data to be fed directly into the automatic train operation systems used on some lines by LU. This will permit these trains to take account of weather conditions appropriately. It will also enable a quicker response to changing leaf fall conditions on other lines and aid the decision on when to deploy the RATs.
Dave was pleased to be able to report that the project is confident of meeting its target schedule, and that he expects it to come in below budget.

This article was written by Chris Parker.

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