Railway "Cab Signalling" systems, particularly those used for Automatic Train Operation (ATO) is another topic I find very interesting.
Modern systems typically use "Balises", often similar to RFID in operation, and a wireless link to the train (occationally WiFi).
While these are great from an operational perspective, I'm much more interested in some of the more creative systems historically used.
I've added a page to capture some notes about a few legacy signalling systems that might otherwise be lost: Legacy Railway Signalling Systems