Since multiple bidirectional tracks are essentially a number of parallel single tracks, bidirectional installations are considered single track installations in this document. Automatic Block Signal Types Automatic Block Signals may be divided into two physical and two logical types. Logically, Automatic Block Signals are divided into advance signaling Automatic Block Signals and non advance signaling Automatic Block Signals. Non advance signaling Automatic Block Signals are always the 2-lamp type. Advance signaling Automatic Block Signals on main lines are always the 3-lamp type.
Some installations vary from the rule.
Automatic Block Signals and Exit Signals on the same line are almost always the same type. Proceed at prescribed speed. The by far most common use of Automatic Block Signals on main lines is as unidirectional doubletrack block signaling, such as it is outlined in the document about Line Block System type DSB 1954 . The Automatic Block Signals are usually approach lit. The green lamps are dimmed when no train is present in the block approaching the signal.