This system, at first adopted only at jmictions and important stations, has upon all important lines been extended over the entire road, the signal stations being- placed in telegraphic communication. This constitutes what is termed the " block-signal " system (26/. 25,fig. 15), and its general intro duction has proved to be exceedingly serviceable in giving achnirable con trol of the movement of trains and in avoiding accidents. By this system the entire line of railway is divided into sections of convenient lengths (say two miles, or even less), each section being commanded by signal stations, one at each end, which are placed in telegraphic commnnication. The object of this disposition is to insure that only one train at a time shall be in any one block-section npon the same track.
Operation of the operation,• the signal-man who at station A has allowed a certain train to enter his block in the direc tion of station B will not permit a second train on the same track to pass his station until he shall have received from the signal-man at signal station B a message informing him that the first train has arrived at sta tion B and has passed into the next block. A second train arriving, meantime, at station A, is halted by the dang-er signal until the proper signal has been received from station B, when it is allowed to proceed. By this systetn, an accident through the entrance of several trains into the same block at a time can arise only from the wilful disobedience of the signal-man or from his misunderstanding of telegraph-signals from adja cent stations. Neglect on his part to attend to the operation of the fixed signal will have no worse result than to bring an approaching train to a standstill at the entrance to his block, since the normal position of the signal is at clanp-„er. To guard against disobedience or inattention to sig nals on the part of the engine-driver, the Pennsylvania Railroad has intro duced at important points a system whereby, when the track is not clear between blocks, the switch is so set as to side-track the train. This pre caution is taken to avoid the greater evil of a possible collision, and is assuredly calculated to make the engine-driver exceedingly careful to observe and obey his signals.
Distant addition to these provisions, so-called " distant signals " are occasionally employed, whose purpose is to notify trains some distance away of their approach to dangerous points. These distant signals are operated mechanically by a \vire communication from the nearest signal-station.
Interlocking Switch and additional security at important stations, junctions, or where a number of distinct lines communicate, it becomes necessary to provide means whereby the' signals and switches can be connected with each other so that both can be operated in conjunction.
The highest type of such combinations is seen in the so-called " interlock ing " switch and signal systems. In these the proper setting of the switches is accomplished before the operation of the safety signal is begun, and the signal is moved to danger before the movement of the switches can be commenced. By no other systems yet devised would it be possible to control the movements of the great number of arriving and departing trains at important railway centres, and their adoption at such points has become indispensable.
Other Signal systems of Saxby and Fanner (p1.2s, fig. 17), the Union Switch and Signal Company, Hall Signal Company, and others of lesser note, are wonderfully ingenious, and perfect in their methods of operation. The movements of arriving and departing trains, which, as at the Cannon Street Station in London, sometimes follow one another at intervals of two or three minutes during certain hours of the day, are con trolled by means of suitable levers, by which the operator shifts the switches and signals. At this station a single operator, with no less than sixty-seven such levers under his charge, receives and despatches during the busiest portion of the day at the rate of thirty-six trains per hour, each of which must be separately signalled, requiring one hundred and eight operations of shifting switches and signals.
important locations, junctions, and the like, the signal-station is usually a tower (pr. 25, jig. 15) raised considerably above the road-level, that the signal-man may be able to command an unob structed view in all directions, and may be isolated from everything calcu lated to divert his attention.
others should be mentioned the SW1 tch-signals, whose object is to inform the engine-driver at some distance of the position of the switches. These signals are targets of various forms and with vari ous distinguishing colors, which exhibit bv their position which way the switch is set 13). At night colored lights are employed for the same purpose. A special form of the switch sig-nal used largely in Austria is the Bender (fii. 2S, figs. ISa, b). In this the light of the lantern is caused, by the intervention of two cone-shaped mirrors, to be reflected upon curved disks of metal, which are so disposed in relation to the mir rors as to be uniformly illuminated.