July 25, 2011

Early Railway Signalling


The year was 1836. A young Englishman by the name of William Cooke was studying anatomy in Germany. One day, he chanced to run across a demonstration of Schilling’s electric telegraph. It then occurred to Cooke that in the electric telegraph—till then looked upon as a mere toy—there lay a device likely to be of the utmost use in railway operations.

On his return home from Germany, Cooke concentrated upon producing instruments adapted for railway use. He also came in contact with Professor Wheatstone, who had been interested in electricity for some years. In June 1837, Cooke and Wheatstone in partnership, took out their patent—which was destined to become epoch-making throughout the world of railways—“for improvement in giving signals and sounding alarms in distant places.”

Indeed the railway signal system proper, with all its elaborate safety devices grew around the Block telegraph system, and it was the invention of the telegraph which really paved the way for progress in signalling throughout the five continents. Till then the safe running of trains was attempted by keeping a ‘time interval’ between the trains. Under this system, trains were allowed to follow one another at a certain interval based on the assumption that the preceding train must have reached the next station at a certain time. This was always a risky assumption fraught with dangerous consequences.

To avoid such accidents, it became apparent that perfect security could be ensured only by providing a definite ‘space interval’ between trains. The development of the electric telegraph provided the necessary means of communication between a pair of stations controlling the dispatch of trains to maintain an adequate space between trains.

Space Interval

This idea of having a space interval between trains using electrical apparatus came to be known as the block system. It is simple in theory. The railway track is divided into a number of sections of a given length, each known as a block. On either side of each block, are fixed block instruments which are installed at the station or cabin. Each cabin is responsible for working a group of signals and points in the vicinity. The block instrument is so designed and interlocked with the track and signals that only one train can use the block at a time. The block sections thus ensure the necessary space interval between trains.

The next improvement in railway signaling is ‘interlocking’. The evolution of interlocking provides an instance of a labour saving device concocted by one of the pioneer signalmen in England. Incidentally, the first railway signal itself—a lighted candle—was the bright idea of an ingenious station master at Hartlepool on one of the pioneer railways of Northern England some 150 years ago. He was tired of leaving his snug retreat in cold weather to furnish a hand-signal to an approaching train. He, therefore, placed a lighted candle on the table in his office, close to the window, to convey the necessary message to the driver regarding the state of the track ahead.

Similarly, a signalman employed at Walford about 1846, had to run from one signal-post to the other for operating signals provided at various points. In order to save his legs he conceived the idea of attaching a weight to the signal and working it by means of a wire which he carried to his hut. This idea was adopted and thus originated the practice of concentrating the levers working points and signals at a convenient spot. A shed was built around the lever-frame to shelter the signalman from the weather. The term ‘box’ or ‘cabin’ arose from this development. The first cabin was set up at Bricklayers Arma Junction in 1856.

British Pattern

In India, where the development of railway signalling has in many respects followed British practice, the system of cabin interlocking was designed and installed by Messrs Saxby & Farmer (India) as early as 1893. The ex-GIP Railway was the first to adopt this system on a large scale on its Bombay – Delhi route. Later other railways followed suit.

Mechanical interlocking has still some disadvantages. The distance over which rod-worked points can be operated is very limited. Hence several cabins are required in a large station yard. And to set up a route for a train, a dozen or more levers have to be operated. This is time consuming and tiresome for the signalman.

This was appreciated by George Westinghouse, the inventor of the air-brake, who applied the principle of this invention to the operation of railway points and signals by compressed air. Since the lever frame was then only necessary for controlling the agency working the points and signals, miniature levers only are required and from this stage developed the power signal frame as we see in our suburban sections.

Introduction of power signalling not only permitted points and signals to be operated in a very short time, but also extended the distance over which they could be worked. Indeed, the whole picture is changing with the development of automatic interlocking, colour light signaling apparatus, remote control and track circuiting.

The block instruments are replaced by illuminated track diagrams on which the position of all trains on the section is indicated. Signal and point positions are electrically repeated on the power frame and electric interlocking circuits prevent point operation while trains are passing over them, or signals being prematurely operated before routes are clear.
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K. R. Vaidyanathan