Because a railroad factory - its plant and train operation - may be spread ojt over thousands of miles and hundreds of communities, it is not surprising that railways have been among the pioneers in the use of improved methods of communication and control.
Another reason for special importance to signalling attached by railways has been that signalling determines the safety of passengers, railwaymen and rolling stock. On the other hand, it is signaling which facilitates the flow of traffic and thereby increases the track carrying capacity.
The earliest railways had no signals and there was no need for signalling. The lines were short and one locomotive made one or two trips a day on a single-track line. Then traffic grew in volume and it became necessary that the train movement should be directed by one way or another. Methods of controlling train operations evolved over many years of trial and error. A common method in the early years was to run trains on a time-interval system; i.e. a train was required to leave station a certain number of minutes behind an earlier train moving in the same direction. It was common on single-track lines to program all operations in accordance with a timetable, which set up all the places trains were to meet and which could not be varied. So the function of this system was to ensure a certain time interval of time between times lest trains should collide. However, this method soon became unsatisfactory because with the increased number of trains and cases of their delays this method could not ensure the interval of space.
The result was that another method was devised which came to be known as the space or distance-interval system. The modification of this method is the modern block system under which the line is divided into short sections of blocks and a fixed colour-light signal is placed at the entrance to each block. The function of the block signal is to prevent a train from entering a specific section of track until the train already in that section has left it.
Most lines in Europe use a manual block, system in which operations are controlled from wayside cabins or towers in conjunction with the wayside signals. Each tower controls a section; a train is not admitted to a section until the train ahead has left the section. Electric interlocking improves this system making it impossible to give a "line clear" signal indication if the section is already occupied by a train.
The basis of much of today's railroad signalling is the automatic block system introduced in 1872, one of the first examples of automation. It uses track circuits that are short-circuited by the wheels and axles of a train, putting the signals to the rear of the train and to the front as well as on single track at the danger aspect. A track circuit is made by the two rails of a section of track, insulated at their ends. Electric current fed into the section at one end flows through a relay at the opposite end. The wheels of the train will then short-circuit the current supply and de-energize the relay.
A logical development of the route-interlocking principle is centralized traffic control, a system in which trains are controlled entirely from a central point through remote operation of switches and signals. The operator sees the track layout in miniature on his control panel and directs the movement of trains over distances from a few miles to many hundreds of miles. Lights on the panel show the location arid progress of all trains at all times. Pushing the buttons the operator changes the position of switches and signals.
In centralized traffic control, track circuiting is essential to ensure that the system always knows where each train is. Switches and signals are operated by coded electrical pulses that reduce the wiring required. Over long distances, centralized traffic control substantially increases track capacity by making more effective use of the trackage.
In spite of the reliable signaling system and highly efficient methods of train operation accidents still occur on railways, particularly at high-speed lines. This happens due to weather conditions and human factor. So railways had to change radically the entire system of signaling. The key to solve the problem of train operation has been found in so-called semi-automatic driving of trains, which is indispensable for safe operation of trains moving at top speed. By semi-automatic
driving of trains is meant the use of locomotive cab signals and automatic train stop devices. The cab signals double the block signals and reduce the speed even if the driver has ignored the warning indication whatever the reason may be Thereby the traffic safety has radically increased. Only a slight further extension of this technique is needed to permit fully automatic operation of the train.
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