Picture a 5p coin. That’s how much contact there is between a train’s wheel and the rail.
It’s the secret to rail’s energy efficiency, giving it such low rolling resistance.
But the other side of that coin is that contamination on this small contact patch can cause low adhesion. And this, in turn, creates problems with acceleration or braking. At times this can lead to delay, infrastructure damage, injury, or derailment.
Solutions available now
There are a range of solutions available to mitigate low adhesion. One key mitigation is fitting sanders to rolling stock. But sand delivery hoses can become clogged by debris and vegetation, and suffer other malfunctions such as nozzle misalignment. All of these reduce their effectiveness.
Last year, a devastating collision between two passenger trains happened in Talerddig, on Wales' Cambrian line. While we await the final Rail Accident Investigation Branch report, we understand from available information that blocked sanders were among the possible contributory factors.
Low adhesion affects our freight customers, too. For instance, at Petteril Bridge in Carlisle, Cumbria, in 2022, five wagons of a freight train derailed when suspected adhesion issues caused one of the wheelsets to develop a very large wheel flat. This resulted in significant damage to infrastructure and a seven-week line closure.
To help operators ensure their sanders are working at their best capacity, RSSB has developed the Good Practice for Sander Maintainability guide. There is also a companion guide on integrating sanders with passenger trains. Angel Trains, Eversholt Rail, and Porterbrook have all recently reported success adopting this guidance to check their fleets in preparation for this autumn.
Projects in the pipeline
And longer-term work on two pieces of research have been started by RSSB in response to an incident at Salisbury Tunnel Junction in 2021 when two trains collided on converging lines. There were no fatalities, but 14 passengers and staff were injured, and significant repairs were required.
The first piece of research is a framework for the assessment of technologies to deliver reliable braking in low adhesion, due in late 2026. It will set out much-needed performance targets and criteria that will allow rolling stock companies, train operators, and infrastructure managers to assess existing and new solutions accurately.
The second project is enabling economic evidence to underpin investment decisions to mitigate adhesion. It will clarify the financial aspects of adhesion, and how benefits are actually realised.
That surface area of contact between the train’s wheel and the rail—the 5p piece—will by its nature continue to be an area of risk. But there are actions operators can take in the short term to address this problem guided by the Good Practice for Sander Maintainability. And in the long term, RSSB is working on research which will further help in this crucial area of rail safety.