I thought people would find the very old signalling system interesting and it's still in use today. "Anderson's Piano" is a set of wires and signals at the Pass of Brander, near Falls of Cruachan in Scotland, that try to detect when there might be a boulder on the track. They're 140 years old, and so far no-one's been able to find a better solution — but they're working on it. After 140 years, this old technology still keeps trains safe - YouTube
