Leaf fall disruption explained

What is low rail adhesion and what are measures combat the problem?

We know that customers get frustrated by delays to Metro services, especially when the delays are caused by low rail adhesion, which happens as a result of leaf fall in the autumn months.

Below is an examination of the issues, why it can hamper Metro services and what is being done to help prevent the problem from occurring.

How can leaf fall cause delays to Metro services?

Low rail adhesion is an international problem that affects train companies right across the world. 

When leaves fall on to the line, particularly in damp or wet weather, the rolling action of passing wheels compresses them, causing a greasy ‘mulch’ to cover the top of the rail.  

This mulch is to rails what ice is to roads. It reduces the adhesion, or ‘co-efficient of friction’ to use the technical term, between the steel train wheels and the rails. Acceleration must be reduced to prevent the wheel from slipping and braking distances need to be extended.  

Because of their lightweight construction and rapid acceleration, Metro trains are particularly susceptible to LRA conditions.  If you’re a regular Metro customer, you can probably sense the wheel “slipping” on a train if you’re travelling through an area that’s affected.

The leaf mulch can also affect the operation of signalling track circuits.

What is being done to prevent low rail adhesion?

  • Extra training has been provided for less experienced Metro drivers to deal with incidents of low rail adhesion.
  • Metro drivers with less than two years’ experience are being provided with mentors to assist in them dealing with rail adhesion issues when they’re in the Metro cab.
  • Tree surgery and vegetation clearance is carried out in to reduce the level of leaf fall in Metro corridors during the autumn months.
  • Vehicles operate in the early hours before trains come into service on key sections of the network, spreading a special cleansing solution on the rails.
  • Nexus uses static Traction Gel Applicators – Five of these units are strategically installed across the network in known areas of low rail adhesion.  These units continually dispense a solution called ‘traction gel’, which is spread along the rails by Metro trains and improves the friction between the wheel and rail interface.
  • Handheld applicators are also used by rapid response teams to address other areas of slipping and in an urgent situation where weather conditions change quickly, silica sand is manually applied to the rails.
  • Quicker reporting of low rail adhesion through the Metro Control Room and daily reviews of specific incidents to target problem areas on the Metro system including proactive monitoring of weather conditions to determine the likely effect on rail head conditions for our drivers.
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