Page:The New International Encyclopædia 1st ed. v. 03.djvu/216

* BLOCK-SIGNAL SYSTEM. 186 BLOCK-SIGNAL SYSTEM. It 'W h m ir 5 J '' f ^te ®' I' n are operated by the signalman at the entrance to the block. In the auto-manual the signals are oper- ated as in the manual or con- trolled manual systems, but are placed in the danger position au- tomatically by the train after the rear car passes them. In the automatic system the operation of the signals is entirety auto- matic, generally through the agency of electricity or a com- bination of electricity and com- pressed air. and no signalmen are required. In train-stati' blocking a special machine is located at the entrance to each block which contains a number of train staffs. The engine-driver receives one of these stall's upon apjiroaching the block, and until this statT is car- ried to the next block and insert- ed in the machine there, no more stafl's can be taken from the first machine. The control between the two machines is maintained by electricity. With any of these systems either absolute blocking or permissive blocking may be practiced. In absolute blocking, two trains are never allowed to be in the same block at the same time. In permissive blocking, a second train may be allowed after a. certain time has ela])sed to en- ter a block before the first train has left it, the second train being compelled to run at decreased speed and observe caution. The accomjianying diagram shows a double-track railway ar- ranged with block-signals. The blocks are the sjjaees between A— B, B— C, and C— D. At A, B, and G there are sign.al-sta- tions, with three signals for each track. These signals are an ad- vance signal, a distant signal, and a home signal. Should all these signals be at safety, the train passes the station into the next block, but should the home signal l)e at danger the train stops. The advance and distant signals are simply to show the engine-driver the condition of the home signal sulliciently in ad- vance to permit him to slow up his train ready for a stop should it be at danger. The form of signal most commonly used is a semaphore arm i)ivted to an iip- right post. When this arm is horizontal the signal is at dan- ger, and when it liangs down in an inclined direction the signal is at safety. For distant signals the end of the semapliore arm is notched. Often disk signals are used instead of the semaphore arm. At night inclosed lights take the place of the semaphore arm. Formerly, a white light meant safety, a red light danger, and a green light cau- tion : but of late years green lias been adopted for safety and red for danger, with yellow or a comliinations of colors for caution. In ail but the automatic systems the signals are thrown to safety (they are so arranged that they always stand at danger until pulled to the safety posi- tion) by pulling the levers resembling the re- versing lever of a locomotive. These levers are located in the signalman's tower, and are con- nected to the signals by means of wires or rods. In the automatic system these signals are o])er- ated by the train itself; the wheels pass over track instruments or levers and set in motion certain mechanical or electrical operations, as a result of which the signals are removed. In manual blocking, which is extensively used in England, each signalman controls the signals at his station. The code signaling between the block stations is generally done by means of electric bells. British lines also use in manual blocking an indicator giving a visual indication of the condi- tion of the block. The block station has. for each block, an indicator connected by an electric cir- cuit with a similar instrument at the other end of the block. On the dial of the indicator appear the words, 'line clear,' 'train on line,' and 'line closed.' By placing the hand of the indicator over any of these phrases, the instrument at the other end gives a like indication. The method of operation of the system as follows: Atten- tion is first called on the bell, and then station A will give to station B wliat is called the 'be ready' signal, consisting of a certain number of strokes on the bell, varying so as to indicate the nature of the train. The man at station B, if the previous train has passed his station and he knows that the block between A and B is clear, repeats this signal. The train is then dis- patched from A, the man at A gives the signal 'train on line,' the man at B acknowledges tliis by moving his own indicator, and, through the connection, the one at A to 'train on line,' and at once gives the 'be ready' signal to station V,. As soon as the train lias passed B the man at B moves his indicator, and therefore the indicator at A, to 'line clear'; and upon this being ac- knowledged by A the indicator is left vertical, signifying 'line closed,' and the operation has been completed so far as that particular train and particular bhiek are concerned. In controlled manual blocking, the manipula- tion of the signals at the entrance to the block is controlled electrically by the signalman at the block station in advance. In what is termed the auto-manual system there is at each block sta- tion a track circuit of at least two or three rail- lengths, which is used for automatically replacing the home signal to danger when the rear end of a train passes that signal. The locking instru- ments are generally placed in front of the signal- man and over the levers with which they operate. Kach block station has (me of these instruments for each block, the two instruments at the oppo- site ends of a block being connected by an elec- tric circuit. The code signaling between the stations is accomplished by bell, as in manual blocking. The mode of operation is as follows: Prior to starting a train from A, the signalman there asks B to unlock the signal at A; and when B does so A throws his signal to safety, allowing the train to proceed toward B. As the rear of the train passes the home signal at A,