They are used, mostly, for monitoring level crossings. Probably they have far too low resolution to be used for monitoring points, electrical measures seem to suit this need much better. Also, at some bigger junctions I've witnessed some cameras apparently used by the Polish Railway Police (SOK = Straz Ochrony Kolei) - they even had their markings on the boxes, and were hung much lower than the LC-monitoring cameras (but the one I've witnessed were connected by ****ty AirLive WLAN access points which were mounted outside the boxes, and as the distance was long and there was no line of sight, I doubted did they actually work, or rather acted as scarecrows).
