This is one of those more abstract items that occasionally catch your eye. I was standing waiting for a train and looked down at the rails for no reason whatsoever. I noticed the end of the third rail. For those that aren’t familiar with rail systems, the third rail is a power rail for electrical trains. There are two main power delivery methods in the UK. The overhead lines provide 25kV of AC power picked up by a pantograph while the third rail provides a nominal 750V of DC collected by a shoe. When the rail switches side, the end will be tapered down. If a train is pulling a lot of power when it joins or leaves the rail, there can be some interesting arcing. I think the patterns on the surface of the rail here were the result of repeated transitions. I know a couple of people that understand this stuff in way more detail than I so maybe they will correct me.