Besides the three colors mentioned, it is customary to mark the entrance to a curve in a subway or tunnel with a blue light, so that the running speed may be reduced to prevent undue wear of the rails.
In locating the home signal on single-track lines, care must be taken that the blocks do not overlap. In other words, the home signal governing the entrance to a block for trains running in one direction must be so placed that a train can go to it without encroaching on the block for trains running in the opposite direction. This should be the arrangement of the home signals at every single-track block station, whether there is a passing siding there or not. The rule that a train leaving a block station has the right to proceed quite up to the home signal of the block station in ad vance should always govern in the locating of home signals. Or, as a general proposition, it may be said of home signals for single-track blocking, that they must be so located that two trains moving in opposite directions to ward a common point can be stopped at that point clear of each other, by those signals. Machine blocking undoubtedly offers the safest method of single-track blocking, and is in considerable use on electric roads. The prin ciple involved is a modification of the English staff system. Staff or tablet machines are used, one being placed at each end of each block, and these machines are so constructed that the withdrawal of a staff or tablet from one machine automatically locks the apparatus so that a second staff or tablet cannot be obtained at either end of the block until the first one is restored to one or other of the two machines of the block. The advantages of the electric train staff are (1) that a train can be sent forward from either end of the section with out awaiting the arrival of the staff from the other end, a frequent source of delay under the old method, and (2) that greater security in working is obtained on account of the impossibility of withdrawing a staff from the machine at either end of the section without the concurrence of the signalman at the other end. A machine is placed at each end of the block, the two machines being connected by an electric circuit.
The rapid development of railway traffic has created a demand for the power signal in block signaling and interlocking. The power signal has also been developed to meet the increasing requirements and is an important factor in the safe and economical operation of railway trains. In blocking or spacing
trains, the power signal in connection with a track circuit automatically indicates the presence of a train in the block, a misplaced switch or a broken rail; in routing or switch ing trains, in connection with electric inter locking, the power signal under control of a leverman, and track circuit also when desired, may be located at any distance from the inter locking tower so that the limits of the inter locking plant are unrestricted; and as a dis tant signal, in connection with a mechanical interlocking plant, the power signal may be located at a sufficient distance from the home signal to afford proper braking distance for high speed trains.
Since the invention of the closed-track circuit in 1872, the power signal has passed through successive stages of development: from the clockwork disc to the electro-pneu matic semaphore; from the electro-pneumatic semaphore to the enclosed disc; from the en closed disc to the electro-gas semaphore; and finally to the highly perfected electric motor semaphore.
The general features of the electro-pneu upper one, according to the general practice, is the or home signal, is painted red and governs the section immediately ahead; . the lower one is the signal, .is painted green (or yellow) and works in harmony with the "danger" signal of the next section. Each . semaphore is operated by a. cylinder three matic system are (l) a rail circuit with its batteries and relays; (2) an air compressor with its pipes to the signal posts; (3) the signal posts themselves with their arms, air cylinders and electrically-controlled valves. Two semaphore signals are placed on a post about 50 feet to the rear of each section. The inches in diameter, fitted with a piston, the rod of which is so connected as to move the signal to its "clear)) position when air pressure from the main pipe line is applied to the pis ton, by means of the electric valves, that be ing the position of the signals when the track is clear for the movement of trains. Each signal blade carries an appropriately colored lens, which covers a light when the blade is in the horizontal position, showing red if it is the upper signal, and green (or yellow) if it is the lower signal. A white (or green) light is exposed when the signal is in position to indicate that the track is clear.