BLOCK-SIGNAL SYSTEM. A method in use on railways to insure increased safety by keeping trains a certain distance apart, by'divid ing the line into blocks or sections, the entrances to which are guarded by appropriate signals for the government of the train. The blocks may be of any reasonable length, depending upon the physical characteristics of the line and the fre quency with which the trains are run. On lines of heavy traffic the blocks are often only several hundred yards long, while on lines with a light traffic they may be several miles in length. The block systems used in modern railway practice are: (1) The manual; (2) the controlled man ual; (3) the auto-manual; (4) the automatic. and (5) the train staff or machine. In manual blocking the signals at the entrance to each block are operated and controlled by the signal man at that station. In controlled manu:il-Idock systems the signals at the entrance of each block are controlled either electrically or mechanically by the signalmen in the next station ahead, but 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. ln the automatic system the operation of the signals is entirely 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-staff 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 staffs upon approaching the block, and until this stiff is car ried to the next block and insert ed in the machine there, no more staffs 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 elapsed 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 accompanying d i a g r a shows a double-track railway ar ranged with block-signals. The blocks are the spaces between A-11, B—C, and C-1). At A, 11, and C there are signal-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 be at danger the train stops. The advance and distant signals are simply to show the engine-driver the condition of the home signal sufficiently 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 pivoted to an up right post. When this arm is horizontal the signal is at dan ger, and when it hangs down in an inclined direction the signal is at safety. For distant signals the end of the semaphore arm is notched. Often disk signals arc 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 has been adopted for safety and red for danger. with yellow or a combinations 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 arc con nected to the signals by means of wires or rods. In the automatic system these signals are oper 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.