Otherwise known as fog signals, these are small explosive devices used to warn drivers to stop immediately as a major hazard is further up the track. On trains, approximately twelve detonators are usually carried in a secure location in the driver's cab. They have an explosive and projectile hazard, though no projection of fragments of appreciable size or range is expected, and crews should maintain a distance of30 metres (this distance will need to be increased if the incident involves a large number of detonators).
A railway fog signal is a small metal device containing a limited quantity of explosive. The device is placed on the running surface of a rail so that any rail-mounted vehicle passing over it would cause it to explode, and in so doing, alert the driver of the vehicle to a hazard on the line ahead.