The third step provides a space interval between trains moving on the same track by dividing the track into a series of con secutive block sections by the use of fixed signals to govern the movement of trains, no train being permitted to enter an occupied block section except under certain restrictions.
Manual Block is a system in which the block signals are oper ated manually upon information by electric communication : first used in England, 1839 and in the United States, 1863.
Controlled Manual Block is a manual block system with an electric control of the signals, used to increase the safety of their operation : first used in England, 1875.
Automatic Block is a system, developed in the United States, defined by the American Railway Association as "a series of consecutive blocks governed by block signals operated by electric, pneumatic or other agency actuated by a train, or by certain conditions affecting the use of a block." When a train enters a block section the signals governing the block automatically move to and remain in the stop position until the train leaves the section controlling the signals. The signals are thus actuated not by human agency but automatically by the train through the medium of a closed track circuit.
The Interstate Commerce Commission has said : "Perhaps no single invention in the history of the development of railway transportation has contributed more toward safety and despatch in that field than the track circuit." The closed track circuit, invented by Dr. William Robinson, was first put in use in the United States in 1872. (Pl. IV., fig. 5a, 5b.) The automatic block system is in general use on the railroads of the United States. Its value as a means for protecting and expediting the movement of trains is receiving world-wide recog nition.
Mechanical Interlocking is a manually operated system which had its inception in the crude machine put in use at Bricklayer's Arms Junction, England, 1843. Thirteen years later a mechanical
machine was put in use in England by John Saxby that included the essential principles of present day interlocking systems. This type of machine was put in use in the United States at Trenton, N. J., 187o. Mechanical interlocking is in extensive use on the railways of Great Britain. In the United States the tendency is toward the use of power interlocking.
Electro-pneumatic Interlocking is a power system, the switches and signals operated by pneumatic power with electrical control of the valves of the pneumatic cylinders : first used in the United States, 1891, and invented by George Westinghouse, the inventor of the air brake. (Pl. IV., fig. 7.) Electric Interlocking is also a power system, the switches and signals operated by electric motors with electrical control. The system now in general use and first used in the United States, 1889, was invented by John D. Taylor. (Pl. IV., fig. 8.) Power Interlocking is used for the operation, as one plant, of what would otherwise require two or more mechanical inter locking plants. The principal terminal stations in the United States are all equipped with power interlocking.
Electro-mechanical Interlocking is a system that was developed for use with automatic block signalling; the switches are operated by mechanical interlocking and the signals by power interlocking: first put in use in the United States in 1906.
Remote Power Switch Machines are used for the operation of outlying main track switches located at too great a distance from either train order, block or interlocking stations to permit of their operation from these stations by mechanical or power inter locking. The operation of switches that would otherwise be hand-thrown by trainmen eliminates making train stops for train men to throw switches. Eliminating train stops decreases train delay.
Automatic Interlocking is a power operated system for junc tions and grade crossings, automatically actuated by a train and not by operators as in other systems of interlocking.