HISTORICAL DEVELOPMENT OF RAILWAY SIGNALING
2.1 ABSOLUTE BLOCK SYSTEM
The principle of the Indian Railways Absolute Block system is quite straight-forward (this can be referred to in Appendix). Only one train should be in any one block section at a time. The Absolute Block system applies on double or multiple lines where trains always use each line in a pre-determined direction and also on single lines but the instruments used on double and single lines are of different types.
The Absolute Block System of working on Double lines
On double lines the cabins on each end of the block sections are equipped with a Block Instrument which is usually of the SGE type mentioned earlier. It has two small windows which show the status of the UP and Down Lines and a round handle known as the Commutator which also has a plunger in the middle. The commutator can be turned through three positions but only by first pressing the plunger in the middle. Pressing the plunger sends one bell beat to the block instrument at the other end. In addition to the block instrument, a bell is also provided which gives a single beat on its gong every time the plunger at the other end is pressed. A block telephone is also provided to enable voice communication between the two block instruments. In addition a buzzer is provided to sound when the leading vehicle of the train passes the Advance Starter and enters the Block Section and an Indicator Lamp is provided which lights up when the Block Commutator becomes Free(explained in following paragraphs).