When a single track running line there needs to be a way to prevent two trains meeting by accident. The simple system of a 'key' that effectively locked the points at the far end was the solution. When the train carrying the key for that section reaches the signal box it is handed over so the section of now clear track can be released. Here you see a key exchange, the train crew have handed over the key for the section just cleared (lower key) and collected the key for the next section. The large hoop made it easier to do this as it passed.
Image size: 6000 x 4000, 10.32Mb | Camera details: NIKON D7200 | Date uploaded: 28/10/2018