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A railway detonator (torpedo in North America) is a coin-sized device that is used as a loud warning signal to train drivers. It is placed on the top of the rail, usually secured with two lead straps, one on each side. When the wheel of the train passes over, it explodes, emitting a loud bang. It was invented in 1841 by English inventor Edward Alfred Cowper.
Typical uses of detonators include:
- A warning, caution or stop signal in dense fog, when signals are difficult to see
- A warning of a train stopped on the line ahead by an incident or accident—the train crew are usually responsible for placing the detonators
- A warning of ongoing engineering works ahead
- When a signaller or other railway employee requires to stop approaching trains in an emergency
- To alert crews working on the rails if a silent runaway train or train carriage is approaching
On a high-speed line, detonators may need to be placed on both rails.
As with all explosives, detonators can become unstable over time and must, therefore, be replaced regularly.
They are triggered by pressure, rather than impact. This makes them safe during transport, as they normally cannot detonate in a bag or storage container.
In the United States
Upon hearing the noise of a torpedo exploding, the engineer reduces speed to 20 mph or less, not resuming its original speed until at least two miles beyond where it encountered the device. They were traditionally used in pairs to ensure that the sound registered with train crews. Torpedoes are essentially obsolete in the U.S. as soundproof construction of modern locomotive cabs renders them useless.
Quoting from the terminology book of the Brotherhood of Railroad Signalmen:
A torpedo is a device which is strapped to the top of a rail. When a train drives over the torpedo, it emits a very loud "bang" which can be heard over the noise of the engine, and signals the engineer to stop immediately. Torpedoes are generally placed by the flagman when protecting a train ahead. Torpedoes are about 2" × 2", red in color, about 3/4" high, and have two lead straps attached, which hold it to the rail. The torpedo has discs inside and are filled with detonating powder. The torpedo was invented about 1874.
In the United Kingdom
Use of detonators is defined by the British Railway Rule Book. Detonators may be used in emergency situations to protect a line on which there is a hazard (for example, a failed train or obstruction which is fouling the running lines). In this case three detonators must be placed 20 yards apart and their detonation indicates the requirement to perform an emergency stop. Detonators may also be used to protect engineering works.
Detonators were used where hazards had to be secured and there was no time for other signaling or if there was a danger that another signal might not be recognizable in time, for example due to fog or snow. To give the emergency signal, three detonators were placed in short succession, with the explosion of a single detonator being a stop signal. Since 1986 detonators have no longer been used on German railways. Only the ICE 3 trains that travel to France still have detonators on board because of French regulations.
The use of detonators has been superseded by radio communications since the early 1950s. In November 2010, the Taiwan Railway Administration deployed 800 detonators for destruction on maintenance tracks. It received media attention, emitting sounds similar to the culturally significant firecrackers.
Today known as audible track warning signals, or audible track warning devices, detonators are used to attract the attention of train crews when track repairs or an obstruction are ahead, or when a hand signaller is acting for a signal.
Detonators are used to attract the attention of train crews in case of:
- Visibility of fixed signal restricted by fog, mist or other circumstance
- obstruction of line, whether by train or otherwise
- protection of push trolleys
- protection of shunting movement outside the station limits during total failure of communication
Many mechanical signal boxes in the UK were equipped with detonator placers that placed detonators on a running line when a lever was operated. The levers were painted a striking white and black chevron pattern, pointing upwards for the "up" line, downwards for the "down" line. In some cases, the placers were fed from a cartridge holding a number of detonators.
According to Military and Civilian Pyrotechnics by Ellern, page 376, FORMULA 155 – Railroad Torpedo, is by mass:
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