BLOCK SIGNAL SYSTEM, the method of protecting railroad trains in motion, through the dividing of the road into sections, or (blocks,* in which but one train is allowed at a time on the same track. The system is operated through signals which have the effect of strict orders to the engineer and conductor in charge of any train.
The real benefits from the block system are increased safety for trains, and economy and increased facility in train operation. Increased safety is gained in three ways: (1) By keeping trains on the road not less than a fixed min imum distance apart, thus reducing to the low est figure the risk of collision, either from trains overtaking each other, or opposing trains meeting on single track; (2) by the responsible corps of signalmen, who have a full view of passing trains and of the tracks for consider able distances, and are in regular communica tion with the track-walkers and other watch men, and are equipped to stop trains in time to save serious trouble from defective train appli ances and road-bed; (3) by demanding of trainmen and enginemen greater vigilance and the habit of being alert at all times for signals, which, under the block system, are compara tively close together. Economy and increased facility in the operation of the train service are ensured by the fact that irregular trains are prevented from doubling up and causing block ades at passing and other points where track room is limited. They also follow from the greater uniformity in the arriving of irregular trains at terminals, which distributes more evenly the yard work of handling cars and power, and thereby also reduces traffic expenses, as the yard work can then be done more eco nomically than when there is a rush at one time and a standstill at another. All signal systems are required to provide for the diverse condi tions existing on a single-track line and the double-track line. In double-track blocking only the movements of trains in following each other have to be provided for, while single track blocking has to do this, and, in addition, guard against trains meeting head-to-head. Therefore, double-track blocking may be dis tinguished by the divisional title of "non-fol lowing block," in contradistinction to that of °non-meeting block," for single-track blocking. While single-track blocking must govern both the non-meeting and the non-following move ments, its main object is to control the non meeting (head-to-head) movements, which are the more important. To accomplish this, it
should provide for holding a train at a passing siding while another train moving in the oppo site direction is approaching that point. There are two principles in single-track blocking which should always be kept in mind. First, all meeting and passing points must be at block stations. Second, for the non-meeting move ments that section of the line between the passing sidings must be operated as one block, regardless of intermediate block stations.
In single-track operation where block sig nals are not in use, trains on the main track are operated and passing points are fixed upon the authority of time-tables and train orders. The problem is to safely move the maximum number of trains in both directions on the same piece of track and so to fix their passing points as to cause the minimum amount of delay. In general, the movement of trains is made from one passing siding to another. The field of railroad signaling is one which has been pro lific of a multitude of safety devices, arranged into many competing systems; but the funda mental wisdom of the establishment of "blocks" has been recognized by all inventors. On the practical side, the railroads have reduced the acceptable systems as proposed to a simple classification of four variations: (1) The manual system; (2) the controlled manual sys tem; (3) the semi-automatic system; (4) the automatic system. In the manual system all signals are made directly by an individual sig nalman at the entrance to each block. As a rule he is in telegraphic or telephonic communi cation with the signal operators at the farther ends of the block stations on either side of him. In the controlled manual system the block is opened by the "clear" signal given by the op erator, but the setting of this signal electrically sets the signal at the farther end of the block at "stop," which, in turn, must be opened to "clear" by the operator at that station. In the semi-automatic system the train entering a block sets the signal at that point at "stop" and holds it at that position until it has passed safely into the next block. It is then released, but the signal operator must set it at "clear" before the next train can enter that block. In the automatic system all signals are operated by the moving trains, without any attention from operators except at switching points.