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 elcetrie hells. 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 hell, and then station A will give to station B what 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 this by moving him own indicator, and, through the connection, the one at A to 'train on line,' and at once gives the 'he ready' signal to station O. As soon as the train has 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 block 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 automatienlly 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. Each block station has one of these instruments for each block, the two instruments at the oppo site ends of a block being eminected by an elec tric circuit. The code signaling between the stations is accomplished by bell, as in manual blocking. The mode of operation is RS 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, that signal is thrown to danger by the local track circuit. As soon as his home signal goes to clan ger, A throws his lever to its normal position of danger, thereby locking it so that the signal can not be thrown to safety again without permis sion from B. A drop-slide indicator or annun ciator forms a part of the electric mechanism, displaying through a slotted opening on the front of the instrument-ease the words 'free.' 'locked,' and 'train in block.' By a system of electric locking working in connection with the indicators, and operated in part by the train through track circuits, B is unable to clear A while there is a train in the block.
Automatic block-signals are usually operated through the agency of electricity alone or a combination of electricity and compressed air by the passage of trains into and out of the block.
A track circuit is generally used for conveying the electric current to the signals. In arranging a track circuit, a battery is placed at the advance end of the block. A relay and a signal are placed at the other end. Each line of rails is connected with a pole of the battery. The current passes along the rails of one side of the track. through the relay. thence along the rails of the other side of the track, thus forming a circuit. The relay controls a local circuit, which works an electro magnet, which in the simple electric system actuates the signal. and in the electro-pneumatic system permits compressed air to operate it. Automatic block-signals are operated on two plans: (1) The 'normal safety' plan, in which the signals always show safety unless the block is obstructed; (2) the 'normal danger' plan, in which the signals always show danger at all times, except when cleared by an approaching train, which can be done only when the block is clear of trains and other obstructions. When the head end of a train enters an automatic elec tric block the current short-circuits, that is. takes the shortest path back to the battery, through the axles of the locomotive or car, and is thereby cut off from the relay, which then breaks the local circuit operating the signal. When the signal is passed by the rear end of the train it goes to danger. In the 'normal safety' plan. when the train passes out of the farther end of the block, the signal again goes to safety. In the 'normal danger' plan the currents are so arranged that the signal is set at danger until it is again cleared by an approaching train.
A train-staff machine consists of an iron pillar with a vertical slot through nearly its entire length, in which is a supply of staffs. The head of the machine, called the `locking-case.' contains the electrical apparatus for manipulating the machine. Only one staff can be withdrawn at a time from the two machines governing a block; and this only with the consent of the signalman at the receiving station, who alone controls the staffs at the sending station. Another staff can not be removed from either machine until the one already out is restored at one or the other end of the block. The following is the method of operation: Before starting a train from A toward B, the situation in the block is ascer tained by means of the bell code. If the block is clear of trains and obstructions. B, when asked, unlocks A's machine. This allows A to withdraw one staff, which is given to the engineman, who, on arrival of the train at B, hands it over to the signalman there to place in his machine. As neither of the machines governing the block can be unlocked while the engineman has his staff, it is of course impossible to furnish a staff for an other train until the first staff has been returned. The train-staff system is essentially a single track system.
The preceding paragraphs describe the typical methods of block-signaling, without describing the apparatus employed. This varies consider ably, and in so many details that it canbe treated clearly only in a special treatise. The actual operation of the block system dates from Decem ber, 1S39, when the Great Western Railway of England began telegraphing the arrival of its trains from station to station for short distances near London. The block system was introduced into the United States by the Pennsylvania Rail road in 1876. Block-signaling is now universal in England, but it has been adopted in full or in part on only a few American railways.