Every Autumn, Network Rail runs Rail Head Treatment Trains (RHTT) to clean leaf debris from the rails. These trains blast the leaf debris, which by now has been firmly rolled onto the rail head by passing trains, with water jets at between 1,000 and 1,500 bar and then deposit sandite – sand with some ground metallic material suspended in a sticky paste. These trains carry a very large quantity of water and sandite, are very effective, but also use capacity on the network.
Are there better techniques for cleaning the rails? Could they be deployed on service trains to avoid the loss of capacity? This is something that the Adhesion Research Group (a sub-group of RSSB’s Vehicle Track System Interface Committee) has been researching for some time. Four alternative methods were presented in a recent webinar hosted by RSSB, and the number of questions asked after the presentations was testament to the interest in the topic.
Four techniques were presented: dry ice, plasma, laser and, lastly, a very surprising solution, given that conventional wisdom says water and leaves are part of the problem, not the solution – improved braking through controlled water addition.
Dry ice
Professor Roger Lewis from Sheffield University described the use of dry ice (solid carbon dioxide CO2) pellets to clean the rail. The principle is similar to the sand or soda blasting used to clean corroded steel; compressed air and the kinetic energy of the pellets bombard the contamination, although in this case, the thermal shock of the cold pellets makes the leaf film more brittle and the sudden expansion of the CO2 gas aids the removal of the leaf film.
It is useful to recall that CO2 does not exist as a liquid at atmospheric pressure, transitioning from solid to gas or vice versa without a liquid state. The supersonic compressed air deals with liquids/moisture and the pellets deal with solid matter. The effect is merely on the surface of the rail and cooling of the rail is virtually non-existent. Even with the treatment train moving 16km/h, the rail head is exposed to the jet for only around 0.005 seconds. There is also no impact on cracks, insulated block joint end posts, ballast (allowing use on switches and crossings) and on the polymer around embedded rail.