How RSSB’s adhesion research projects lead to practical benefits for rail

The GB rail industry is predominantly affected by low adhesion during the autumn. However, the rail network can still encounter low adhesion issues throughout the rest of the year. Low adhesion issues can impact operational performance and cause safety incidents such as Signals Passed at Danger (SPADs), station overruns, buffer stop collisions, and train collisions. Contamination can also insulate the electrical connection between the wheel and track, leading to Wrong Side Track Circuit Failures.

The consequences of adhesion

Notable adhesion-related incidents include the Salisbury Tunnel Junction collision on 31 October 2021, in which there was the collision and derailment of two passenger trains, resulting in injuries to the driver and 13 passengers. Another example is the station overrun at Stonegate on 8 November 2010, where a passenger train failed to stop at Stonegate station, and ran for a further 2.45 miles with the emergency brakes continuously applied. Evidence showed that the train failed to stop at Stonegate because there was almost certainly no sand in the sand hoppers at the leading end of the train.

The impact on the GB rail industry and wider society in total is estimated to cost £355m per autumn.

Solutions

The industry has developed numerous solutions over the years to mitigate and alleviate the low adhesion problem. Examples include friction modifiers, such as sand and traction gel, to increase adhesion levels; high-pressure water jetting to clean the rail head; and Wheel Slide Protection to prevent wheel flats. 

RSSB has invested in promising solutions to enable a step change in adhesion management. This has been made possible through the cross-industry Adhesion Research Group which steers work on this topic ensuring wide industry awareness and ownership, and funding received from the Department for Transport to conduct research that enables a safe and efficient railway. 

Rail head cleaning

While water-jetting can be effective at removing rail head contamination, it uses vast quantities of water and has significant costs and limitations. Numerous initiatives for an alternative to water jetting are under development. 

One example is the use of dry ice. This alternative was initiated in its early feasibility and demonstration stages through RSSB funding, and following its promising results has been pursued by Cryogrip with support from Northern Trains and Network Rail.

Dry ice (cryogenic) pellets can be used to shock the contaminant layer that forms on rails, causing it to crack and de-bond. Dry ice leaves no residue behind on the rail head and is also non-conductive. This means it can be used in environments where water-based solutions could present safety risks, such as around equipment with electric, pneumatic, or hydraulic components. The Cryogrip solution offers modular and scalable systems which have been fitted to a range of railway vehicles, including passenger trains and rail head treatment trains. 

Further passenger train trials are planned for November 2023 on the Edinburgh Suburban Line, and the Goole Line with Northern Trains. The latter trial will include speeds up to 75mph. A longer term objective is to combine dry ice pellet technology with an Artificial Intelligence friction estimation tool to enable ‘smart’ rail head cleaning. 

Sanding

Most trains in Great Britain are fitted with sanders that discharge a fixed rate of sand. Following conclusive RSSB research, operators of longer vehicle formation have started using the second sander, which was fitted but not operational, to help reduce the risk of adhesion related incidents.

Enhanced sanding is also now an option. Enhanced sanding delivers variable rates of sand in a speed-dependant manner, meaning sand delivery is optimised at both high and low speeds. Comprehensive full-scale testing by RSSB has shown that using two variable rate sanders in series provides significant stopping benefits in low adhesion. In particular, a double variable rate sanders (DVRS) configuration can halve stopping distances on a four-car train compared to single fixed rate sanders.

RSSB’s research enabled a national DVRS business case to be undertaken by the Rail Delivery Group, and retrofitting of DVRS is now underway on several passenger fleets, funded by the Performance Innovation Fund. This includes Northern Trains, ScotRail, and South Western Railway.

Controlled water addition

Studies have shown contaminated rails provide the most significant slipperiness when damp, while dry or heavily wetted tracks provide better friction. Using a controlled amount of water to create ‘rainy day’ braking conditions can provide a step-change improvement in both braking and traction.

Following initial feasibility and demonstration projects funded by the RSSB-led Innovation Programme, Water-Trak Ltd was formed as a joint venture between CoCatalyst Ltd and SCB Associates Ltd, to enable commercial exploitation of this technology. 

Water-Trak has now recorded approximately 80,000 miles of data, with retrofitting on several of Northern Trains Class 319s and Class 170s. In full service braking, a Water-Trak fitted Class 319 (Water-Trak combined with single variable rate sanders) demonstrated a 40% reduction in stopping distance compared to the control group (Class 319 fitted with single variable rate sanders).