Railroad signaling and operations
Railway signaling is a system used to direct railway traffic and keep trains clear of each other at all times. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which makes it difficult to quickly stop when encountering an obstacle. In the UK, the Regulation of Railways Act 1889 introduced a series of requirements on matters such as the implementation of interlocked block signaling and other safety measures as a direct result of the Armagh rail disaster in that year.
Most forms of train control involve movement authority being passed from those responsible for each section of a rail network (e.g., a signalman or stationmaster) to the train crew. In the U.S., the set of rules and the physical equipment used to accomplish this determine what is known as the method of operation.
Most forms of train control involve movement authority being passed from those responsible for each section of a rail network (e.g., a signalman or stationmaster) to the train crew. In the U.S., the set of rules and the physical equipment used to accomplish this determine what is known as the method of operation.
Copper ball
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In the early years, when railroading was in its infancy, one train might be the only one running on a specific track for a day or more. During this time, rail signaling reflected the form being used in England and parts of Western Europe. These countries were using steam power, a copper ball and a flagpole. The copper ball was raised when a train was fueled up, passengers and freight were loaded, and the track was properly switched. This “highball” was the “ready to go” signal.
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semaphore
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Semaphore is one of the earliest forms of fixed railway signals. These signals display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'. Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely used form of mechanical signal.
The first railway semaphore signal was erected by Charles Hutton Gregory on the London and Croydon Railway at New Cross, southeast London, about 1842 on the newly enlarged layout also accommodating the South Eastern Railway. In the U.S., semaphores were employed as train order signals, with the purpose of indicating to engineers whether they should stop to receive a telegraphed order. |
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block system
As rail traffic increased, multiple trains began to use the same track. As a result, there became a sudden need for more stringent traffic control, and the concept of a “block” was developed.
The blocking system broke a line of track into smaller segments able to be controlled with signals. This meant that at a certain interval along a given track, these early chain and ball signals would be placed to secure a section of track along with an operator to maintain the signal. While one train cleared a section of track, another train waited at the signal for it to clear. Then when everything was cleared, the ball signal was raised and the train proceeded.
The blocking system broke a line of track into smaller segments able to be controlled with signals. This meant that at a certain interval along a given track, these early chain and ball signals would be placed to secure a section of track along with an operator to maintain the signal. While one train cleared a section of track, another train waited at the signal for it to clear. Then when everything was cleared, the ball signal was raised and the train proceeded.
History of the block system
On double tracked railway lines, which enabled trains to travel in one direction on each track, it was necessary to space trains far enough apart to ensure that they could not collide. In the very early days of railways, men were employed to stand at intervals ("blocks") along the line with a stopwatch and use hand signals to inform train drivers that a train had passed a certain number of minutes previously. This was called "time interval working". If a train had passed very recently, the following train was expected to slow down to allow more space to develop.
The watchmen had no way of knowing whether a train had cleared the line ahead, so if a preceding train stopped for any reason, the crew of a following train would have no way of knowing unless it was clearly visible. As a result, accidents were common in the early days of railways.
With the invention of the electrical telegraph, it became possible for staff at a station to send a message (usually a specific number of rings on a bell) to confirm that a train had passed and that a specific block was clear. This was called the "absolute block system".
Fixed mechanical signals began to replace hand signals from the 1830s. When a train passed into a block, a signalman would protect that block by setting its signal to 'danger'. When an 'all clear' message was received, the signalman would move the signal into the 'clear' position.
The absolute block system came into use gradually during the 1850s and 1860s and became mandatory in the United States in the early 1890’s. This required block signaling for all passenger railways, together with interlocking, both of which form the basis of modern signaling practice today.
The watchmen had no way of knowing whether a train had cleared the line ahead, so if a preceding train stopped for any reason, the crew of a following train would have no way of knowing unless it was clearly visible. As a result, accidents were common in the early days of railways.
With the invention of the electrical telegraph, it became possible for staff at a station to send a message (usually a specific number of rings on a bell) to confirm that a train had passed and that a specific block was clear. This was called the "absolute block system".
Fixed mechanical signals began to replace hand signals from the 1830s. When a train passed into a block, a signalman would protect that block by setting its signal to 'danger'. When an 'all clear' message was received, the signalman would move the signal into the 'clear' position.
The absolute block system came into use gradually during the 1850s and 1860s and became mandatory in the United States in the early 1890’s. This required block signaling for all passenger railways, together with interlocking, both of which form the basis of modern signaling practice today.
coded track circuit
The dawning of reliable electricity led to the invention of a coded track circuit which used common principles of conductivity. A box of circuits and electromagnets called a “relay” was placed at each end of a section of track. Each rail was then electrified by a supplied current. At the ends of each section of track, or “block”, a strip of insulation was placed between the rails so that the next block could have its own circuit and not interfere with the circuits of surrounding blocks.
When a train passed into an electrified block, the circuit from one rail would travel over the steel axles of the train to the other rail and create a connection. The relays would then detect this loss of electricity and a series of electromagnets would become demagnetized. This created a new circuit that then directed power to the railroad signal which rotated a pivot and illuminated a lens from green to the red. Thus creating electrified block territories that are still used today.
When a train passed into an electrified block, the circuit from one rail would travel over the steel axles of the train to the other rail and create a connection. The relays would then detect this loss of electricity and a series of electromagnets would become demagnetized. This created a new circuit that then directed power to the railroad signal which rotated a pivot and illuminated a lens from green to the red. Thus creating electrified block territories that are still used today.
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Some track territory featured more than one track, with some tracks going in all different directions, such as yards, crossings with other railroads, or high traffic regions. This was where manned signal operators remained necessary. Manned interlocking towers were used on the railroad to control these points. Each tower was given two letters to identify itself on the telegraph wire. The letters usually involved some relation to the name of the town but were ordered so they weren't confused with other letter codes used on the telegraph.
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In the mid 20th-century, Centralized Traffic Control system (or CTC) was developed. This was a large console with a series of lines depicting tracks, switches and other miscellaneous track structures. At each track switch depicted on the console, there was a small light bulb and a small lever. When the light bulb was lit, that meant a train was occupying that “block” and if the train's destination required transfer to another track, the operator, miles away could simply turn the lever or push a button and instantly a signal created an impulse in a relay box that then in turn operated a motor and switched the track. With this amazing new technology, manned signal towers were no longer needed, and the railroad companies began to demolish some of these towers and installed traffic control consoles in centralized locations.
timetable operation
The simplest form of operation, at least in terms of equipment, is to run the railroad system according to a timetable. Every train crew understands and adheres to a fixed schedule. Trains may only run on each track section at a scheduled time, during which they have 'possession' and no other train may use the same section.
When trains run in opposite directions on a single-track railroad, meeting points ("meets") are scheduled, at which each train must wait for the other at a passing place. Neither train is permitted to move before the other has arrived. In the US the display of two green flags (green lights at night) is an indication that another train is following the first and the waiting train must wait for the next train to pass. In addition, the train carrying the flags gives eight blasts on the whistle as it approaches. The waiting train must return eight blasts before the flag carrying train may proceed.
When trains run in opposite directions on a single-track railroad, meeting points ("meets") are scheduled, at which each train must wait for the other at a passing place. Neither train is permitted to move before the other has arrived. In the US the display of two green flags (green lights at night) is an indication that another train is following the first and the waiting train must wait for the next train to pass. In addition, the train carrying the flags gives eight blasts on the whistle as it approaches. The waiting train must return eight blasts before the flag carrying train may proceed.
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The timetable system has several disadvantages. First, there is no positive confirmation that the track ahead is clear, only that it is scheduled to be clear. The system does not allow for engine failures and other such problems, but the timetable is set up so that there should be sufficient time between trains for the crew of a failed or delayed train to walk far enough to set warning flags, flares, and detonators* to alert any other train crew.
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A detonator is a railroad or track signaling device. It is a small dynamite charge that is usually wrapped in red paper with metal straps to hold it firmly in place on a railroad rail. When a locomotive’s wheel comes into contact with the torpedo, the weight of the engine sets off the charge creating a loud bang to signal the conductor to stop the train. Today, with the use of two-way radios, railroads do not use track torpedoes as much as they did in the past.
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A second problem is the system's inflexibility. Trains cannot be added, delayed, or rescheduled without advance notice.
A third problem is a corollary of the second: the system is inefficient. To provide flexibility, the timetable must give trains a broad allocation of time to allow for delays, so the line is not in the possession of each train for longer than is otherwise necessary.
Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train. Timetable operation was the normal mode of operation in North America in the early days of the railroad.
A third problem is a corollary of the second: the system is inefficient. To provide flexibility, the timetable must give trains a broad allocation of time to allow for delays, so the line is not in the possession of each train for longer than is otherwise necessary.
Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train. Timetable operation was the normal mode of operation in North America in the early days of the railroad.
Restricted speed operation
Restricted Speed or Line of Sight Operation implies that trains will control themselves so that they can come to a stop within half the range of the engineer's vision or line of sight. There are many types of Restricted Speed Operations with slight variations, such as Yard Limit rules or Industrial Track rules depending on specific operational circumstances. They all have the same basic working theory – that two trains, approaching head on, will each be able to come to a complete stop upon seeing one another.
Restricted speed operation generally works with a maximum speed of 15 or 20 mph and a typical speed governed by length of train and visual conditions.
Restricted speed operation generally works with a maximum speed of 15 or 20 mph and a typical speed governed by length of train and visual conditions.
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