The need of indicating the conditions of the road to trains came with the increasing traffic and speed. As these conditions developed in England before they did here, the first steps were taken in that country. In 1834 the Liverpool & Manchester introduced the first system of fixed signals, consisting of an upright post with a rotating disk at its top, showing red for danger and the absence of indication by day and a white light by night for clear. On the opening of the Great Western Railway this method was improved. Experiments by Messrs. Chappe, the inventors of optical telegraphy, showed that under certain conditions of illumination the color of any body would disappear. This demonstrated that the form, and not the color, of the day signal could be relied on. It was also found that a long, narrow surface could be seen further as projected against the horizon or landscape than the same area in a square or circle. Making use of these results, Sir Charles Gregory, in 1841, designed and erected at New Cross the first semaphore signal. There was no communication between stations; each signalman displaying his signal at danger after the passage of a train until a certain time had elapsed, when it was cleared. The only information conveyed to the engineman was that the preceding train had passed the station at least the required time before him.
The failure or inability to act with sufficient promptness at the display of the danger position, and the consequent collisions, led to the installation of additional signals to give advance information to the engineman of the position of the signal he was to obey. Thus we have clearly portrayed the inception of the present block and caution signals.
Mr. C. V. Walker, of the Southwestern Company, introduced the "Bell Code," which was the first audible method of communication between signal stations. The same year Mr. Tyer supplemented this with electric visual signals, the object being to give the operator indication of the signal having been received and given, and at all times to show the exact position of the signal itself. This suggested the space interval between trains, in place of the time interval, making signal indications definite. In 1858 the positive block system was established in England, based on the space interval system.
Making use of telegraph communication, Mr. Ashbel Welch, Chief Engineer of the United New Jersey Canal and Railroad Company, devised and installed during 1863 and 1864 the first block system of signals in this country, on the double-track line between Philadelphia and New Brunswick. Signal stations were suitably spaced, and at each station a signal was provided, visible as far as possible each way. The signal itself was a white board by day and a white light by night, indicating "clear," shown through a glass aperture two feet in diameter in front of the block signal box. For the "danger" indication a red screen fell to cover the white board or light. On a train's passing a station the signalman released the screen, which fell by gravity, and did not raise it until advised by telegraph that the preceding train had passed the next station, thereby maintaining a space interval. Thus was evolved the telegraph block system, still generally used, with modifications of apparatus and signals, on lines of light traffic. Elaborations of this system were later installed following more closely the English practice, perhaps reaching the most complete development upon the New Haven and New York Central lines, where it is still in use. Notwithstanding numerous improvements in apparatus, the same practice of fixing a positive space interval by means of communication between block stations still holds. The addition of track circuits for locking and indicating purposes and interlocking between stations, more fully effected by the introduction of the "Coleman block instrument," in 1896, has thus evolved the controlled manual block system as now used.
AUTOMATIC SIGNALS.
In 1867 Thomas S. Hall patented an electric signal and alarm bell, used in connection with a switch or drawbridge. Its shortcoming lay in the fact that a break in the circuit or failure of the latter gave no danger indication. To correct this a closed circuit was necessary, although more expensive. In 1870 Mr. Wm. Robinson devised the plan of having the circuit closed at the point of danger, if conditions were favorable, and opened a short distance in advance of the signal. The wheels of the approaching train depressed a lever, which closed the circuit and cleared the signal, unless interrupted at the point of danger. Subsequent modifications were made, whereby the circuit once completed remained so through the agency of an electromagnet, and reopened when the train passed out of that portion of the track governed by the signal.
In 1871 Mr. Hall put in operation the first automatic electric block system, on the New York & Harlem Railroad, between the Grand Central Station and Mott Haven Junction. It was normal "safety." The wheels of a passing train striking a lever completed a circuit, which put the signal to danger, after the train, and held it so until the succeeding signal went to danger, when a separate circuit was completed, which released the former signal, allowing it to return to clear.
The disadvantage in having the wheels of a train strike a lever to complete the circuit led Mr. F. L. Pope to experiment. After a successful attempt in transmitting an electric circuit through an ordinary track with fishplate joints, he made a signal test at East Cambridge, Mass. A section of track was insulated from the rest, with a wire circuit, including a battery and electromagnet for operating the signal, fastened at either end to the opposite rails. The metal wheels and axles completed the circuit, throwing the signal to danger against following trains. A detent served to keep the circuit closed until the next signal was reached, when a separate circuit released the detent, permitting the signal to clear.
In 1879 this system was put in service, and, with some alterations, still remains in some localities.