C. RAILWAY CONSTRUCTION (cont.)
Railway Signals: Signals; Block System; Interlocking System.
The earliest passenger railways were opened without any fixed signals. Flags and disks, elevated on posts and pillars, where first employed, in various forms, and were worked on various codes. Sir Charles Hutton Gregory, about the year 1841, designed and erected at New Cross station, on the Croydon Railway, the semaphore signal, an adaption of the old form of semaphore used for telegraphing over short distances. This was the most important step ever taken in the development of railway signaling. The semaphore has been almost universally adopted for fixed signaling on railways. There are two arms, to the right and to the left, to command trains arriving in either direction. The arm is turned out horizontally, in a position perpendicular to the post, to signal danger; diagonally downwards at an angle of about 45°as a signal of caution; and it is turned home, disappearing within the post, when the line is right for the approach of a train. But the general practice now (1885) is to work the semaphore in two positions only,at danger and at caution. It is thus always in sight, and its position can be identified without hesitation. To make the signal system safe there must be clear definition and strict enforcement of the duties of the attendant; good men must be selected at adequate pay, and they must have convenient, warm, well-fitted lodges, with ample window-space, within which they may keep a constant watch over the line without exposure to weather. At junctions and other important signal-stations the lodges should be raised some height above the surface, to give perfect supervision in every direction and prevent distraction. At night the place of semaphores or disks is supplied by large and powerful lamps with reflectors, capable of showing lights of three colours,a white light, a blue or green light, and a red light, signifying respectively safety, caution, danger; or, as in general practice, two lights only are shownred and green.
Signalling has been a subject of much controversy, and has been divided into two main systems. In "negative" signalling the normal position is that of caution, or that of safety, as the practice may be, and the signal is only turned on to danger when specially required for the protection of the station on the line. On the other hand, the "positive" system presupposes the normal state of the signal to be that of danger, so that, if the signalman neglects his duty to lower the semaphore when the station is clear for the passage of an approaching train, the train is bound to stop. The positive system has long been in successful operation at all large and important junctions. In such a situation the use of a stringent code is manifestly conducive to the greatest degree of safety, as by the unavoidable intersections of the lines of rails there are many chances of collision. The positive system has been merged in what is known as the block system of signaling,that is to say, the positive system has been on most railways extended to every station on the lines in combination with telegraphic signalling. The best, perhaps the only, safeguard against error on the part of the persons in charge of trains is to be found in the adoption of the absolute block system, and of means for enabling engine-drivers to observe signals well in advance. The absolute block system consists in dividing the line of railway into intervals of convenient lengths, and by means of telegraphic and fixed signals allowing only one train at a time on any single length of single way. The signalman at station A does not send a second train to station B until he receives a signal from station B that the first train has arrived there; meantime the signal at A stands at danger until the man at B singles the arrival of the train at B. Under the "permissive block" system it is simply permitted to signalman B to block signalman A in the event of anything occurring at station B that may render that course advisable. But supposing that a train has just left station A, then the message from B comes too late to enable signalman A to prevent the train from running into the obstruction at B. The permissive system has been well tried on the principal railways, and is preferred on some lines because it admits of trains being passed on one after another with greater rapidity than on the absolute block system. But it does not afford much protection, and it is now generally preferred to work on the absolute block system, and, for the purpose of doing so effectually, to erect intermediate stations on the lines of constant traffic, so as to provide shorter intervals for blockings and obviate the delay incidental to unduly long intervals. The average distance apart of passenger stations is, say, 3 miles, but the distance of signal-stations, whilst it seldom exceeds 4 miles, is frequently only a quarter of a mile, and the average interval may be taken as 1 1/2 miles. Distant signalsthat is, signals placed at a distance in advance of points of danger worked by wire communication from the signal-boxwere, it is believed, first introduced on the North British Railway at Meadowbank station near Edinburgh in 1846, after the opening of the Hawick branch. In 1852 the Great Northern Railway was completely fitted with distant signals of the semaphore type. Distant signals are occasionally fixed at 1500 yards distance; but beyond 800 yards their action is uncertain, and it is checked by repeaterelectric or mechanicalby which, by way of confirmation, the signal is returned to the signalman.
As railway junctions were multiplied it became apparent, not only that distant signals were to be provided for distinct lines, but that concerted action should be established between signals and switches. They are said to be connected when they are simply coupled together and are moved simultaneously. They are said to be interlocked when the necessary movement of the switches is completed before that of the signal to safety is commenced; and, conversely, the movement of the signal to danger is completed before the movement of the switches can be commenced. This is the fundamental principle of the interlocking system of signaling now generally practiced. By the combination of the absolute block system and the interlocking system the greatest possible numbers of trains are enabled to travel over one pair of rails in a given time. At Cannon Street station, at the busiest time of the day, eighteen trains arrive and eighteen depart within an hour; 108 operations of shifting switches and signals, by means of sixty-seven levers or handles, have to be performed in that time. On the North London Railway, at Liverpool Street station, 250 trains pass over the same rails in a day of nineteen hours, averaging only four minutes between trains; frequently only two minutes elapse. The number of trains daily using Moorgate Street station on the Metropolitan Railway is more than 770, involving twice as many movements of engines1540 movementson four lines of way in nineteen hours, and every movement is separately signalled. This, of course, could not be performed without the aid of electric instruments, to enable the signalmen to communicate with each other, and to have a constant record on the faces of the instruments to show what is being done.
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