Modern railroading simply makes the same demand of the telephone that it makes of the telegraph--that it keep the probability of safety high. It makes the same demand of the men who maintain the signals, the track, the bridges, and other portions of the right-of-way. Let us consider them in the passing of an instant. You know the signals along the line of the railroad--those gaunt, uncanny things that spell danger or safety to the men in the engine-cabs. A little while ago, we stood beside a man in the sun-filled tower of a great railroad terminal and watched him operate the most complicated switch and signal system in the land, watched him with the crooking of a finger upon the lever of an electric machine raise this blade, lower that, as he made new paths for the many trains, coming and going. A plant of that sort is known as the interlocking. In its simplest form, it will guard a junction between two single tracks. The mast of the signal will rise, according to standard custom, at the right of the track in the direction of travel, and there will probably be two semaphore blades, the upper of which guards and signals the straight main-line or "superior" track, the lower, the diverging branch, known as the "inferior" track. The blade raised--automatically showing a red light--indicates that the main line is closed to the engineer. "Stop!" "Danger!" are the words it tells him. The blade lowered, a green light is automatically displayed, and the engineer knows that he can go ahead at full speed on the main line. The road is clear for him. The lower blade gives similar indications for the branch diverging line. Normally, both blades stand at "stop" and "danger," and the one guarding the line for which the train is destined, is dropped only on the approach of the train, itself. In fact, to facilitate the movement of trains, these guarding signals--known to the signal experts as "home signals"--are generally interlocked with "distant signals" several hundred feet down the line, on which blades indicating the diverging tracks forecast the story that the "home signal" is to tell the engineer. The blade raised--by night displaying a white or safety signal--on the "distant signal" indicates that the line it guards is blocked at the "home signal," and that the engineer must be prepared to bring his train to a full stop. Dropped--showing the green safety light--that particular line is open and ready, and the engineer can be prepared to pass the junction without a very great diminution of speed.