Fog Signal Switch

Shown is a replica Igniter Fog Signal, Mark 1 also known as a Fog Signal Igniter

This was used to derail trains. The fog signal ignitor was the first method of setting off the charges, and later the SOE's No. 5 Pressure Switch with Extension Rod was used. Cortex (British, white or the early type was lead colour) or Primacord (US, orange) explosive cord was normally taped to a detonator within the fuse snout. If the objective was to destroy the locomotive, the cord would run to the detonator within high explosive which was attached to the rail close by or buried under a sleeper right there or maybe 8 or 10 feet away in the direction from which the train is coming from. It would be a very large charge, 10 or 20 pounds. If exploded under the centre of the locomotive it would lift it into the air and blow it apart.  

The more common tactic was derailment because small charges could be used and set quickly. Derailment usually meant a number of cars went off the track too, more track was damaged, but the engine and cars were not always damaged heavily. Derailments were caused using two charges placed 15 or 20 feet beyond the switch. The cord went to the first, then on to the second. Charges were taped or wired to the side of the rail, three feet apart. If only a three foot section of rail is cut away, an engine will head into that gap and go off. The reason is train wheels are not flat on the bottom, perpendicular to the wheel. They are beveled so the wheels put some outward pressure on the rails. This keeps the train centred, not floating left and right, banging the respective wheel flanges against the rails. So if a piece of rail is missing, off the engine goes. 

For fog signals, they put the fuse snout aiming outward so the wheel flange didn't cut the cord right before the wheel ran over the signal. One downside to fog signals is foot guards could see them. Also the Germans started putting small metal scoops on the front of locomotives, which scraped the signals off.