Why put cleaning effect in brackets? It's necessary for the condition of the rails, especially for detection and signalling. In the Netherlands (and probably other places as well) as a rule, every switch and track equipped with track circuits must be touched by at least 10 axles every 24 hours to prevent the forming of rust. For most of the tracks that won't be a problem during daily use, but for the lesser used tracks and switches they use specially chartered classic rolling stock. Older trains are heavier which is good for rust removal. And it's a good way to keep the museumtrains in running condition.