Learning about train Signals Part 2

Instructions

Permissive signals and automatic block signaling Permissive signals respond to track occupancy. The invention of the track circuit in 1872 by William Robinson permitted automatic block signals. Most simply, a low voltage battery current runs up one rail and down the other. As long as current flows from the battery through one rail to the relay and back through the other rail, the relay remains energized and routes current from another battery to the green lamp of a clear signal. But when a train is present, the circuit is short circuited by the steel wheels and axles of the cars; a broken rail or an open switch will also break the circuit. As a result the relay is opened and the signal displays red. The system is arranged so the failure of any component results in a red indication or a dark signal (which must be interpreted as the most restrictive aspect that signal can display). The Automatic Block Signal (ABS) system permitted much shorter blocks than the manual block system and had the effect of increasing line capacity. The distant signal proved just as useful for the automatic block system as for manual block. It became common practice with lower-quadrant semaphores to place the home signal for the immediate block and the distant signal for the next block on the same mast. An automatic block signal system is composed of a series of block signals. Except where a current of traffic has been established on a track and Rule 251 movement authority is in effect, permissive signals and ABS do not authorize the use of, or movement on, the main track. Their role is that of protective overlay: an added layer of protection placed on the operation. If a train violates its movement authority, it and a conflicting movement will both soon encounter red permissive signals, resulting in much embarrassment, but not an accident. As a practical matter, Centralized Traffic Control (CTC) territory always includes ABS as a protective overlay. Territory controlled by Track Warrrant Control (TWC), Direct Traffic Control (DTC), or Form D Control System (DCS) may or may not include an ABS overlay; this commonly depends on the traffic volume of the particular line. Herein lies a unique circumstance: A train can be at a green signal in TWC territory and be unable to proceed, unless it also has a track warrant to do so. Likewise it can be facing a red permissive signal and yet proceed at restricted speed if it holds the proper TWC (or DTC, DCS, etc.) movement authority. Block signals are often “approach lit” to conserve bulbs and electricity. They remain dark until the approach of an oncoming train is detected, and are illuminated only when a train is in the block that the signal faces. The circuitry is more complicated, but lamps last longer and power consumption is less.