Back in the days when locomotives running on branch lines had no sophisticated 'cab signalling', and only oil-lit track side semaphore signals for drivers to rely on for safety, fog could be a real menace. On dark windless November nights it sometimes thickened up over the moors so badly that you couldn't see the signals if you were standing underneath them on the ground, let alone observe them from the footplate of a moving engine.
In conditions like this 'fog men' were called out by the signalman - actually members of the local permanent way gang doing extra hours of duty. It would be their job to place detonators on the rails at key signals - usually the 'distant' and 'home' signals - and then wait there and only remove them when the signal was in the 'clear' position. This meant that if a locomotive passed and the signal was at danger there would be a heck of a 'bang!' to warn the driver. But it was a cheerless job on a cold autumn or winter night. Sometimes there was a small lean-to hut they could use, and a brazier - if they were lucky.
These fog detonators were large percussion caps about 2" (5cm) in diameter and half an inch (1cm) thick and they had thin lead ribbon 'ties' that held them secure on the top of the rail when in place. Normally they were stored hung in a string on an interior wall of the signal box.
Bowes signal box had a 'high-tech' version of this practice installed in the 1950's - detonators placed outside the signal box by each line which were moved on and off the rail worked by levers in the cabin frame. I think it was the only box over Stainmore to have this facility.
Detonators were seldom triggered in earnest, and the explosives they contained had a limited 'shelf life'. Every so often a replacement batch of detonators would arrive from Darlington and the time-honoured safe way of disposing of the out-of-date ones was to spread them around the tracks in the 'yard' and have the engine from the local pick-up goods set them all off. For a minute or two it sounded like an artillery barrage - always a popular event with local railwaymen!