Page:Popular Science Monthly Volume 4.djvu/603

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and is about to proceed on its journey to E, and that an ordinary train has preceded it a quarter of an hour, which would allow about time enough for it to "shunt" or go off into the siding at D. When the train has passed the signal-box at C, the signal-man there telegraphs to the one at B that the line is clear, which means that there is no train on the "up" line between stations B and C. Directly the express referred to arrives at B, the signal-man there, if he has received the signal "all clear," allows the train to pass him, and at once telegraphs to C that there is a train on the "up" line. The C signal-man, if he has received a signal from D of line clear, allows the express to pass him also, but, if not, he exhibits his signal accordingly. We will now suppose that the express is yet between B and C, and that another train, approaching in the same direction, whistles to the man at B for leave to go on. This is refused until the C signal-man telegraphs that the line is clear. The same plan is carried out at every signal-box the train has to pass, of whatever nature it may be, whether "express," running at the rate of fifty-five miles an hour, or "goods," steaming along easily at a pace of twenty-five. It will thus be seen that, however great the traffic, it can be conducted with almost absolute safety, the only difference being that, with a very large number of trains per hour, the signal-boxes are placed nearer together, as on the Metropolitan or Underground Railroad of London, which has as near as possible one thousand trains passing over its system in the course of every twenty-four hours. No accident of any importance has ever occurred on this line.

But there are other objects in our signal-box besides plungers and little bells to attract attention. Four very noticeable toys—I use the term advisedly, for they struck me at once as being particularly suggestive of liliputian railroads, and dolls' houses, and toy signal-men—are the miniature electric semaphores used for instructing the signal-man as to setting the semaphore-signals on the line for the guidance of drivers of locomotives. Having stated the principle of the "block" system to be that no two following trains are to be allowed to proceed in the same direction upon the same section of line at the same time, it follows that a danger-signal must be exhibited and maintained at the station or depot from which a train has departed until it has been cleared out of the section of the line over which it is traveling. To do this effectually necessitates that this signal should be under the control of the signal-man toward whom the train is approaching; and no accident, mechanical or electrical, should be allowed to remove this signal until the train has arrived. The signals used on most of the English lines of railroad to guide the driver are, the raising and lowering of a semaphore arm to denote "danger" and "all clear." If it were possible to work these huge out-door signals by electricity, the system would be perfect; but, inasmuch as the power of electricity is circumscribed, the production of force sufficient