During the prevalence of fogs, when neither signal-posts nor lights are of any use, detonating signals are frequently employed, which are affixed to the rails, and exploded by the iron tread of the advancing locomotive. All guards, policemen, and pointsmen who are not appointed to stations, and all enginemen, gatemen, gangers and platelayers, and tunnel-men, are provided with packets of these signals, which they are required always to have ready for use whilst on duty; and every engine, on passing over one of these signals, is to be immediately stopped, and the guards are to protect their train by sending back and placing a similar signal on the line behind them every two hundred yards, to the distance of six hundred yards; the train may then proceed slowly to the place of obstruction. When these detonating signals were first invented, it was resolved to ascertain whether they acted efficiently, and especially whether the noise they produced was sufficient to be distinctly heard by the engine driver. One of them was accordingly fixed to the rails on a particular line by the authority of the company, and in due time the train having passed over it, reached its destination. Here the engine driver and his colleague were found to be in a state of great alarm, in consequence of a supposed attack being made on them by an assassin, who, they said, lay down beside the line of rails on which they had passed, and deliberately fired at them. The efficiency of the means having thus been tested, the apprehensions of the enginemen were removed, though there was at first evident mortification manifested that they had been made the subjects of such a successful experiment.

—F. S. Williams’s Our Iron Roads.