the mystery of snowy interspaces

The fallen snow in the end february 2004 offered an odd perspective on the two railtracks between Aachen West and Aachen Hbf. As snow was nearly completely molten away, there were only some small bars of it, positioned in an ostentatious way. About 25 times there were four sleeper interspaces without snow followed by one filled with it (as shown in the picture). Who knows what caused this effect?

Solution by Bob (a.k.a. DHG500): One solution to your question on the snowy interspaces might be that the small stripe of remaining snow is situated right on the lids/covers of the signalling cable-canals. These concrete or polythene canals are dug in, just alongside the track and are covered with separate covers/lids. After period with severe snow, the air trapped in these canals remain cold longer then the outside/atmospheric temperature (exposed to sunlight, whereas the canals stay dark). Therefore the snow on these covers will last longer and where one cover touches the next cover, some of this cold air will emerge and cools/freezes some of the surrounding area as well. The wider stripes you see every fifth sleeper are located on these intersections between each cover.