How does it work?
It is well understood that water plays a key role in the adhesion between a train wheel and the rail head. A dry rail, even with contamination, gives excellent adhesion - with friction coefficients as high as 0.3 to 0.5. A fully wetted contaminated rail, for example during rain, still provides a good level of adhesion with friction coefficients above 0.1. However, a small amount of moisture combined with contaminants such as iron oxides or leaf matter can lead to very poor adhesion. The friction coefficient can drop as low as 0.01, leading to serious problems for train braking and traction. For more information see our blog post.Water-Trak works by disrupting this critical amount of water to create “rainy day” braking conditions. When low adhesion conditions are detected by the train Wheel Slide Prevention (WSP) system, water is dispensed in front of the leading axle of the train. This wetting of the rails, increases the adhesion level almost instantaneously, enabling an improved braking performance.
In a similar way, the system can also provide improved traction. If the WSP system detects wheel slip, then water can be dispensed onto the rails, providing an increase in adhesion for the driven wheels as they pass over the wetted rail.
How much water does it need?
Testing has shown that a relatively small amount of water is required to generate a step-change improvement in adhesion: 1 to 4 ml of water per metre of rail (less than a teaspoon per metre of rail). For example,a train braking from 50mph would require less than two litres of water to be dispensed. During Autumn 2022, the four Water-Trak trains operating in passenger service used around 1300 litres of water in total. As a reference, one Network Rail Railhead Treatment Train (RHTT) dispenses around 5000 litres in one hour.What about the following train?
Water-trak will only dispense water when the train encounters an area of low adhesion. Depending on the weather conditions and elapsed time, a following train may also benefit from this added water. During our testing we have discovered that even adding a small amount of water during braking accelerates the cleaning of the rails, greatly benefiting the adhesion of following trains.How can leaves cause such disruption?
It sounds ridiculous that leaves could cause such disruption to trains and in fact the "leaves on the line" label is somewhat misleading. The problem arises due to a black "teflon like" layer forming on the railhead. Leaves fall directly onto the railhead or are sucked up by the aerodynamics of the passing train. The very heavy weight of multiple passes of the train compresses the leaves into a very hard, black leaf layer. When this layer is damp, perhaps due to dew or light rain, it becomes very slippy and this is what causes the low adhesion problems.What is "wet rail"?
Although low adhesion problems occur mostly in Autumn, they are not all caused by leaves. In over 50% of low adhesion events that are recorded by the industry, no leaf contamination is evident on the rail head. This situation is known as "wet rail" and occurs when the rail head has a small amout of moisture (perhaps caused by dew, fog or very light rain), combined with some other contamination such as iron oxides, coal dust or soot. Adding more water will again disrupt this "critical amount of water" and move towards "rainy day" conditions.Does it affect the current on-train low adhesion mitigations such as WSP and sanding?
During testing in 2020 we were able to show that water addition has a positive effect on Wheel Slide Prevention system operation. These tests also showed significantly improved train deceleration when sanding and water were used together. For more information please see our blog post.What about frost and ice?
The freezing/melting point of water is a function of pressure - as pressures increases, the freezing point temperature reduces. The pressure at the contact point between a train wheel and railhead is very high and this will prevent ice forming or if there is already ice it will immediately turn to water. Ice does not usually cause low adhesion problems for trains for this reason. We have also incorporated features within the Water-Trak system to make it robust to freezing conditions.What is the "magic" ingredient in the water?
We are often asked "What have you added to the water to get this improvement in adhesion?"; it is perhaps counter intuitive that adding water should increase adhesion - the reverse is probably expected. However, it is simply water, there are no additives; Water-Trak replicates nature's solution by creating rainy day conditions on the railhead.Will this work with real leaf contamination?
Unlike some lab testing that is conducted using washing up liquid, all of our testing has been conducted on realistic rail contamination that has been created using real leaves or paper tape that is an industry standard for creating low adhesion conditions. Water-Trak was successfully trialled on the mainline during the peak leaf fall period in Autumn 2020. Water-Trak went into passenger train service in Autumn 2021 and has successfully completed two autumn seasons, operating on routes known to experience leaf fall related problems. There are currently five trains equipped with Water-Trak in service with Northern trains, creating a large body of evidence. See our detailed autumn reports from 2021 and 2022.What is the advantage of water addition compared to sanding?
All trains in the UK have sanding systems which are used as a mitigation for low adhesion. When a train detects low adhesion, usually through activation of the wheel slide prevention (WSP) system, sand is dropped in front of the third axle. When the sand is correctly deployed into the wheel rail contact point, this gives an effective increase in adhesion for the third and to a lesser extent following axles.Sanding does however, suffer from a number of problems:
The sand is non-conductive and can affect the track circuit (the method used currently to detect a train). This is the reason that sanding is not deployed in front of the first axle (losing any benefit of improved braking from the first two axles - this is particularly an issue for shorter trains). The amount of sand must also be limited to avoid impact on the track circuit.
Sand is abrasive and can increase the wear of the wheel and rail. It may also damage rail infrastructure such as switches, points and ballast (sand can lead to "wet bed" problems by disrupting the drainage).
Sand can only be effective if it correctly enters the contact point between the wheel and the rail. This requires the sand to be kept dry (clogged sand will not dispense correctly), the nozzles to be correctly aligned and for wind and aerodynamic effects to not blow the sand away. All of these areas are known to cause problems with current sanders. Our trials have shown that water addition can enhance retention of sand on the rail, increasing the amount of sand entering the contact point.
Maintaining full sand hoppers requires a complete sand supply infrastructure including plastic packaging, transportation and storage. The processes involved have a high carbon footprint, take time and energy and add to the running costs for the train operators.
In contrast water does not suffer these problems:
Water is conductive and will actually improve the track circuit. This allows the water to be dispensed at the front axle, giving benefit to all the axles of the train. The water will also clean contaminants from the wheels.
Water will not clog! A fairly wide spray pattern is used and water will of course flow across the surface of the rail, making the dispenser less sensitive to the accuracy of the nozzle position. The water spray pattern is carefully designed to minimise the impact of cross winds, ensuring that the water will contact the rail head as required.
Water is relatively cheap and readily available. Train depots already have water filling facilities.