fog signal
A small explosive charge (detonator) which can be placed on a railway line in fog, to be detonated by a train passing over it as a warning signal to the driver.
fog signal: audible warning detonated under a train's wheels
A fog signal is a small metal capsule containing a primary explosive charge, designed to be clamped or placed directly on a railway rail. When a train wheel passes over it, the weight of the wheel detonates the charge, producing a sharp explosive report audible to the locomotive crew. In conditions of poor visibility, fog, or heavy rain where visual signals are unreliable, fog signals serve as a backup method to alert drivers of track hazards, speed restrictions, or signals that may not be visible.
The standard construction consists of a metal cup or disc, typically about 50 millimetres in diameter, containing a small quantity of fulminate of mercury or lead azide as the primary explosive. A metal clip or band secures the signal firmly to the top of the rail head so that the explosive charge sits directly in the path of an approaching wheel. When struck with sufficient force, the detonation produces a loud crack audible at considerable distance, penetrating noise that was difficult to ignore in the steam era and remains useful as a secondary alarm.
Placement and working life
Fog signals are positioned on the line at specific locations: typically about 200 metres before a danger signal, warning signal, or section closure where a train must reduce speed or stop. A single signal is often deemed insufficient for secure working, so pairs or triplets are placed at intervals to ensure multiple detonations as a train approaches. Once placed, a fog signal remains active until struck; the detonator does not ignite or deteriorate from weather or age, and uncovered signals can sit on rails for weeks if train traffic does not pass over them. This durability made fog signals valuable in remote locations or on branch lines with infrequent services.
The primary limitation is reliability under poor placement or wheel slip conditions. A signal placed too far to the side of the wheel path may not fire. On greasy rails, a wheel may slip slightly without sufficient impact pressure to detonate the charge. Modern practice increasingly relies on electronic cab signalling systems and wayside detection equipment rather than fog signals, yet they remain part of emergency procedures on many railways and are still manufactured and stockpiled for contingency use, particularly on heritage lines and in regions where electrification or modern signalling infrastructure has not been fully deployed.
The term 'fog signal' reflects the equipment's original purpose in nineteenth-century fog-prone railway networks, particularly in Britain. Alternative names include 'detonator', 'track torpedo', and 'rail cracker', though 'detonator' can also refer to the explosive device itself rather than the complete track-mounted assembly. The device remains notable in railway folklore and accident investigation, as fog signals can provide documentary evidence of when a train was running and which direction it was moving, since the detonation leaves a carbon mark or residue on the wheel face.