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Danger Train Signals On
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Railway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding. Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop within sighting distance of the driver. In the UK, the Regulation of Railways Act 1889 introduced a series of requirements on matters such as the implementation of interlocked block signalling 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. The set of rules and the physical equipment used to accomplish this determine what is known as the method of working (UK), method of operation (US) or safeworking (Aus.). Not all these methods require the use of physical signals and some systems are specific to single track railways. The earliest rail cars were first hauled by horses or mules. A mounted flagman on a horse preceded some early trains. Hand and arm signals were used to direct the “train drivers”. Foggy and poor-visibility conditions gave rise to flags and lanterns. Wayside signalling dates back as far as 1832, and used elevated flags or balls that could be seen from afar.The simplest form of operation, at least in terms of equipment, is to run the system according to a timetable. A fixed schedule is drawn up with which every train crew must be familiar. Trains may only run on each section of track at their scheduled time, during which they have 'possession' and no other train is permitted to use the same section. When trains are running 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. 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 or torpedoes (UK and US terminology, respectively) to alert any other train crew. 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 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.Timetable and train order
See also: Train order operation With the advent of the telegraph in 1851, a more sophisticated system became possible because this provided a means whereby messages could be transmitted ahead of the trains. The telegraph allows the dissemination of any timetable changes, known as train orders. These allow the cancellation, rescheduling and addition of train services.North American practice meant that train crews generally received their orders at the next station at which they stopped, or were sometimes handed up to a locomotive 'on the run' via a long staff. Train orders allowed dispatchers to set up meets at sidings, force a train to wait in a siding for a priority train to pass, and to maintain at le /'
' ast one block spacing between trains going the same direction. Timetable and train order operation was commonly used on American railroads until the 1960s, including some quite large operations such as the Wabash Railroad and the Nickel Plate Road. Train order traffic control was used in Canada until the late 1980s on the Algoma Central Railway and some spurs of the Canadian Pacific Railway.Timetable and train order was not used widely outside North America, and has been phased out in favor of radio dispatch on many light-traffic lines and electronic signals on high-traffic lines. More details of North American operating methods is given below. A similar method, known as 'Telegraph and Crossing Order' was used on some busy single lines in the UK during the 19th century. However, a series of head-on collisions resulted from authority to proceed being wrongly given or misunderstood by the train crew - the worst of which was the collision between Norwich and Brundall, Norfolk, in 1874. As a result, the system was phased out in favour of token systems. This eliminated the danger of ambiguous or conflicting instructions being given because token systems rely on objects to give authority, rather than verbal or written instructions; whereas it is very difficult to completely prevent conflicting orders being given, it is relatively simple to prevent conflicting tokens being handed out.Trains cannot collide with each other if they are not permitted to occupy the same section of track at the same time, so railway lines are divided into sections known as blocks. In normal circumstances, only one train is permitted in each block at a time. This principle forms the basis of most railway safety systems.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 (originally called 'policemen', and is the origin of UK signalmen being referred to as "bob", "bobby" or "officer", when train-crew are speaking to them via a signal telephone) 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 more or less than 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 or signal box 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. These were originally worked locally, but it later became normal practice to operate all the signals on a particular block with levers grouped together in a signal box. 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. Railway infrastructure on the hill-country main line, Sri Lanka, including a gantry of semaphore signals The absolute block system came into use gradually during the 1850s and 1860s and became mandatory in the United Kingdom after Parliament passed legislation in 1889 following a number of accidents, most notably the Armagh rail disaster. This required block signalling for all passenger railways, together with interlocking, both of which form the basis of modern signalling practice today. Similar legislation was passed by the United States around the same time. Not all blocks are controlled using fixed signals. On some single track railways in the UK, particularly those with low usage, it is common to use token systems that rely on the train driver's physical possession of a unique token as authority to occupy the line, normally in addition to fixed signals. Entering and leaving a manually controlled block Before allowing a train to enter a block, a signalman must be certain that it is not already occupied. When a train leaves a block, he must inform the signalman controlling entry to the block. Even if the signalman receives advice that the previous train has left a block, he is usually required to seek permission from the next signal box to admit the next train. When a train arrives at the end of a block section, before the signalman sends the message that the train has arrived, he must be able to see the end-of-train marker on the back of the last vehicle. This ensures that no part of the train has become detached and remains within the section. The end of train marker might be a coloured disc (usually red) by day or a coloured oil or electric lamp (again, usually red). If a train has entered the next block before the signalman sees that the disc or lamp is missing, he will ask the next signal box to stop the train and investigate.Route signalling and speed signalling Signalling of British origin generally conforms to the principle of route signalling. Most railway systems around the world, however, use what is known as speed signalling. Under route signalling, a driver is informed which route the train will take beyond each signal (unless only one route is possible). This is achieved by a route indicator attached to the signal. The driver uses his route knowledge, reinforced by speed restriction signs fixed at the lineside, to drive the train at the correct speed for the route to be taken. This method has the disadvantage that the driver may be unfamiliar with a route onto which he has been diverted due to some emergency condition. Several accidents have been caused by this alone. For this reason, in the UK drivers are only allowed to drive on routes that they have been trained on and must regularly travel over the lesser used diversionary routes to keep their route knowledge up to date.
Under speed signalling, the driver is not informed which route the train will take, but the signal aspect informs him at what speed he may proceed. Speed signalling requires a far greater range of signal aspects than route signalling, but less dependence is placed on drivers' route knowledge.
Approach release When the train is routed towards a diverging route that must be taken at a speed significantly less than the mainline speed, the driver must be given adequate prior warning. Under 'route signalling', the aspects necessary to control speed do not exist, so a system known as approach release is employed. This involves holding the junction signal at a restrictive aspect (typically 'stop') in order that the signals on the approach show the correct sequence of caution aspects. The driver will brake in accordance with the caution aspect, without necessarily being aware that the diverging route has in fact been set. As the train approaches the junction signal, its aspect may clear to whatever aspect the current track occupancy ahead will permit. Where the turnout speed is the same, or nearly the same, as the mainline speed, approach release is unnecessary.
Under "speed signalling", the signals approaching the divergence will display aspects appropriate to control the trains speed, so no 'approach release' is required.
May 31st, 2013
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