Signaling

Types of signals. Railroad signals are used to inform the train crew of track conditions ahead and to tell it how to operate the train. Methods of controlling train operations evolved over many years of trial and error. The earliest form of railroad signal was simply a flag by day or a lamp at night. The first movable signal was a revolving board, introduced in the 1830s, followed in 1841 by the semaphore signal. One early type of American signal consisted of a large ball that was hoisted to the top of a pole to inform the engineman that he might proceed.

The semaphore signal was nearly universal until the early years of the 20th century, when it began to be replaced by the colour-light signal, which uses powerful electric lights to display its aspects. The number of lights and the range of aspects available from one signal can vary depending on its purpose. For example, additional lights may be installed to the left or right of the main lights to warn the driver of divergence ahead from the through track. Red (stop or danger), yellow (warning), and green (track clear) have the same significance worldwide, but they also are used in combination of two colours. Colour-light signaling is now standard on almost all lines of the world’s principal railways.

Automated systems. The railroad is divided into sections called blocks, and at the block divisions there are signals which indicate to an oncoming train whether it may enter that section of track. The important principle which is followed is that only one train is allowed into each block section at a time. The basis of much of today’s railroad signaling is the automatic block system, introduced in 1872 and one of the first examples of automation. It uses track circuits that are short-circuited by the wheels and axles of a train, putting the signals to the rear of the train at the danger aspect. A track circuit is made by the two rails of a section of track, insulated at their ends. Electric current, fed into the section at one end, flows through a relay at the opposite end. The wheels of the train will then short circuit the current supply and de-energize the relay. Thus, in automatic signaling, the train itself activates the danger signals behind its path. It then automatically changes them to warning or clear signals as it leaves each block.

In a conventional automatic block system, permissible headway between trains is determined by the fixed length of each block section and is invariable. Modern electronics has made possible a so-called “moving block” system, in which block length is determined not by fixed ground distance but by the relative speed and distance from each other of successive trains. In a typical moving block system, track devices transmit to receivers on each train continuous coded data on the status of trains ahead. Apparatus on a train compares this data with the train’s own location and speed, projects a safe stopping distance ahead, and continuously calculates maximum speed for maintenance of that headway. A moving block system increases the throughput of trains over one track in a given period of time.

To ensure observance of restrictive signals, a basic form of automatic train control is used by many major railroads. This system gives the driver audible (sound) and visual indication in his cab of signals ahead and applies the brakes of the train automatically if he passes a yellow warning signal without reducing speed in readiness to stop at the next danger signal.

Interlocking and routing. Interlocking of switches and signals at crossings and junctions prevents the displaying of a clear signal for one route when clearance has already been given to a train on a conflicting route.

The functions of track circuits have been multiplied by electronics. Centralized traffic control (CTC) is a system in which the signaling of a long section of railway is undertaken from one control point.Early CTC centres con­tained one panoramic diagram of the area under control, with small lights to show the exact location of any trains on the line, and push buttons which could operate the switches and signals and set up non-conflicting routes for any train movement. In the most recent CTC centres the overall display is retained, but dispatchers have colour video screens portraying close-ups of the areas under their specific control. CTC operation saves time and money by reducing train stops and increasing efficiency and safety.