Trains may run up to, but not beyond, home or advance signals at "danger." They must not proceed until the clear signal or proper authority to pass the °danger° signal is given. When a distant signal is at "caution" trains may pass it under control, prepared to stop at the home signal. Home, advance and distant signals at "safety" may be passed with out reducing schedule speed. A train that is to be held at an advance signal must first be brought to a stop at the home ,signal. It may then be allowed to proceed, under control, to the advance signal. The clearing of a home signal, in any case, gives authority to proceed to the advance signal only. To avoid the necessity of placing the home signal at "clear" for a train that is to be held at the advance signal, an auxiliary signal may be used. This is known as a "calling on" signal, and is a short semaphore blade fixed upon the home signal post below the home signal blade. When in the "safety" position this signal gives a train authority to pass the home signal at "danger" and proceed under control to the advance signal..
Automatic Blocking.— Automatic block signals are operated through the agency of electricity or of a combination of electricity and compressed air, by the passage of trains into and out of the block. A track circuit is generally used in automatic blocking, for con veying the electric current to the signals. In arranging a track circuit, a gravity battery is commonly used, and isplaced at the advance end of the block. A relay and the 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 form ing a circuit. The relay controls a local cir cuit which works an electromagnet, which in the simple electric system actuates the sig nal, and in the electro-pneumaticvstem per mits compressed air to actuate it. The electric power may be generated by a dynamo. The use of storage batteries in electro-pneumatic interlocking permits of a closer regularity of the quantity and pressure of the current, and results in considerable economy in operation.
Automatic block signals are operated under two plans: (1) the normal-safety plan, that in which the signals always show "clear" unless the block is obstructed either by a train or otherwise; (2) the normal-danger plan, that in which the signals are at "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.
Automatic block signals are applicable to junctions, railroad crossings and drawbridges if manual interlocking is used. Separate signals are provided for the interlocking. The same rules apply to the block signals here as else where. If the interlocking and block signals both show "clear," trains may proceed with speed. At a junction, where the route diverges, the block signals will show when there is no train in the block ahead on the main line, and the interlocking signal is lowered for a train to continue on the main line, or when there is no train in the block ahead on the branch line and the interlocking signal for the branch is lowered.
The ordinary automatic block system pro vides protection for following movements, but for opposing movements it fails to meet the requirements of an ideal system for the reason that the block for opposing movements does not extend from siding to siding, an inherent defect in the system.
In the absolute permissive block system the block for opposing movements is from siding to siding and the signals governing entrance to this block are in all cases absolute or stop and-stay signals, hence the term "absolute" in the name of the system. For following move ments the block between sidings is divided into two or more sub-blocks, as traffic conditions may require, and movements into these sub blocks are governed by intermediate signals which may be absolute (stop-and-stay), or per missive (stop-and-proceed) signals. Ordinarily, intermediate signals are permissive or stop-and proceed signals as used on double track, hence the term "permissive" in the name of the system.
Absolute Permissive Block S ystem.— This system of automatic block signals is adapted to meet the exacting requirements of single-track signaling. On double track, train movements on the same track are in the same direction. The fundamental requisite of double track signaling is to so arrange the signals as to provide a space interval between trains cor responding to a given time interval. For ex ample, if the maximum traffic consists of trains operated at one-mn,ute intervals and at a speed of 60 miles per hour, there would be a space interval between trains of one mile (less the train length). This space interval is divided into block sections so arranged as to permit trains to follow each other under adequate sig nal protection.
On single track, train movements are in both directions. The fundamental requisites are two, one for following movements as in double track and the other for opposing move ments. To accommodate following train movements, the block sections are arranged to permit trains to follow as in double-track sig naling. For opposing train movements the block sections must be arranged to prevent the display of signal indications that would permit opposing trains to enter the same block section at the same time. These block sections should extend from one passing siding to the next. This, however, is not a feature of the ordinary type of automatic signaling, but is one of the important features of the absolute permissive T movements are governed by permissive ' als, the block extending from signal to signal; 3) at meeting and passing points maximum signa pro tection provides an efficient check against failure to observe meeting and passing points;(4) flagging is reduced to a minimum, as following trains operate as in double track; (5) a reliable caution signal indication is displayed for every stop indication; (6) the minimum distance is reduced 30 per cent between following trains running under clear signals as compared with block system. Another requisite is that a re liable caution indication be displayed for each stop indication. This is a vital point at passing sidings where opposing trains approaching each other must have ample warning of a signal in the stop position.