Learning about train Signals Part 1

Instructions

Read below to learn more about why signal systems are needed, how they are used, and the types of signals in service on North America’s main-line railroads. There is more to understanding railroad wayside signals than simply “green means go, red means stop.” To appreciate what the signals you see along the track are telling you, you first have to grasp a few basic concepts. Railroad traffic control boils down to three situations: trains running in the same direction on the same track; trains running in opposite directions on the same track; and trains running on two tracks that intersect. One of the first types of traffic control was the timetable, which specifies where each train should be at a given time. To cope with unforeseen circumstances – delays, breakdowns, extra trains – the dispatcher issued train orders, which supplemented and superseded the timetable. Another type of traffic control is to divide the line into sections or blocks and permit only one train in a block. Initially the system required a person at each end of each block to record trains entering and leaving the blocks, and a way for them to communicate with each other. The manual block system still exists on many lines but with different names and different technology – the communication is by radio. Manual block signal indications were simple: stop, the block is occupied; proceed, the block is clear; and proceed at restricted speed prepared to stop short of a preceding train. Because trains had to approach each station prepared to stop, the “distant” signal was developed to give an advance indication of whether a stop was required at the “home” signal. WHAT SIGNALS DO: TWO TYPES, TWO FUNCTIONS There are two types of signals, permissive and absolute. You can tell if a signal is permissive or absolute by looking at it. The rules vary by railroad, but permissive signals usually have a number plate on the base of the mast on which they’re mounted, or the letter “I”. Absolute signals do not have the number plate or will have the letter “A”. The location of the signal is also often a clue to its type. Signals have two purposes, protection and control. Permissive signals only provide protection. Absolute signals provide both control and protection. Functionally, the basic difference between permissive and absolute signals is the most restrictive type of indication each can display. A red permissive signal means stop and proceed. After stopping (which is no longer required on some railroads) the train can proceed at restricted speed until a more favorable signal is reached. (Restricted speed is usually 15 or 20 mph, being prepared to stop upon sighting any obstruction or problem.) But a red absolute signal means stop – and stay stopped. The only way to pass a red absolute signal is with the dispatcher’s verbal permission or, in rare circumstances, under the protection of a flagman. (To see a glossary of signal terms, see the link at the bottom of this story.)