Using Tribo-Chemistry Analysis to Understand Low Adhesion in the Wheel-Rail Contact

Low adhesion between wheel and rail is a recurrent problem for the rail industry.
Low adhesion can lead to wheel slides and slips during acceleration and deceleration,
which can cause large amounts of damage to the wheel and rail as well as causing
safety issues and delays if a train cannot accelerate or decelerate when necessary.
Adhesion in the wheel-rail contact is affected by the third body layer which is present
in the contact patch between wheel and rail. It is composed naturally from steel
wear debris and iron oxides, but often contains other contaminants such as organic
matter, ballast dust, soil and grease. Different environmental conditions such as
temperature, precipitation and humidity change the properties of this third body
layer and therefore change adhesion conditions on the railway.
Low adhesion has been well documented throughout the autumn season due to
organic contamination, but also takes place throughout the year when no visible
contamination is seen on the railhead, known as the “wet-rail” phenomenon. It is
thought to occur when there are low levels of water on the railhead, formed by dew,
mist or light rain, rather than heavy rain.
The conditions and mechanisms that cause the phenomenon are not fully understood.
Low adhesion does not occur very often and under what is likely to be a
narrow window of conditions, which means that it can be difficult to simulate and
study.
The aim of this work was to use a combination of tribology and chemistry to better
understand the cause of low adhesion throughout the year, known as the wet-rail
phenomenon. It investigated low adhesion conditions that occur all year round,
initially focusing on the role of iron oxide in low adhesion as it has previously been
hypothesised that oxides could play a major role in the wet-rail phenomenon.
Testing was carried out over a range of conditions on three different tribological test
rigs to attempt to simulate low adhesion due to the wet-rail phenomenon, which
produced valuable information about the causes of low adhesion. It was found that,
under certain conditions, a combination of iron oxides and water could cause low
adhesion in a simulated wheel-rail contact. Test methods were designed to simulate
the wet-rail phenomenon, which can be used as a platform to better understand the
causes of low adhesion and to test future mitigation methods.