TEXT 1. Railway Switches or Points

The terms “points” and “switches” (the latter is more common in American English) are used to denote devices, usually comprising tapered metal blades or tongues, for setting alternative routes of running rails. In the somewhat more general sense of a curved track leading from one track to another, the term “turnout” is also used in this connection. The most common form of switch is the split switch in which one rail of the main track and the inner rail of turnout are continuous. There are various other types of railway switches, sometimes embodying a combination of two split switches (double turnout). Where two tracks simply cross each other without provision for trains being routed from one track to another, the term “crossing” is usually employed. In some cases, however, more particularly when the two intersecting tracks form a small angle with each other, the crossing may take the form of a so-called crossing switch (also known as “slip points”).

The operation of all these devices is similar in principle and can best be explained with reference to the ordinary split switch. When the straight tongue is in contact with one rail (I) and the curved tongue is not in contact with another one (II), the switch is set for running straight ahead on the main track. When the switch is set so as to divert a train coming from the left on to the turnout track, the curved tongue is swung into contact with the rail II and the curved tongue is now no longer in contact with the rail I. The point of intersection of the inner rails is called the “frog” of the switch. It is usually in the form of a V-shaped unit. As a safeguard against derailment the rails opposite the frog are provided with guard rails, and the frog itself is assisted initially (i.e., at its point) by wing rails to carry the weight of the wheels passing over it.

TEXT 2. Railway Signal Box

Mechanical System

The switches (points) and signals on a section of railway track are worked by controls accommodated in a signal box (or tower). On lines carrying scheduled train services these controls are operated in accordance with predetermined timetables.

In the signal box are pulleys, each connected to an operating lever. When the signalman moves the lever, the pulley is rotated a certain amount, depending on the desired switch position. A catch secures the lever in position. A steel wire rope passes round the pulley. The end of this rope is attached to the switch actuating mechanism or to operating wheel of the main signal. The tensioning device keeps the wire rope constantly taut. Now when the signalman swings the operating lever, the actuating wheel of the switch mechanism is rotated through a certain angle by the wire rope, so that the actuating lever (connected to the wheel) is likewise swung about its pivot and thus shifts the tongues of the switch to the desired position. The main signal is similarly worked, the arm of the signal being moved by a rod attached to the actuating wheel.

 

Electric System

Operating a mechanical signal box involves the exertion of considerable physical force by the signalman. For this reason in modern installations the points and signals are worked by small electric motors. On a control desk in the signal box each switch has its own control key (electric switch). Also, there are similar keys for controlling the signals. The signal box also contains, at eye level, an illuminated diagram showing the track layout and all the switches, crossings and signals of the section of the railway line. The switch and signal positions are indicated on this diagram by means of small colour lights. A glance at the panel also shows whether any particular track is free or occupied by a train. An illuminated diagram of this kind greatly facilitates the signalman’s task.

A further development, providing even greater convenience and reliability of operation, is the “track plan” signal box. In this arrangement the signalman’s control desk itself is laid out as a track plan showing all the signals and switches. Each of these is provided with a key or push-button by means of which the corresponding signal or switch on the track can be operated. The tracks themselves are represented on the control desk by small illuminated compartments. Unoccupied tracks and track sections which at any particular moment are not in use as a train route remain dark, i.e., not lighted up. The switches in the track plan are additionally marked by yellow lamps installed in slots. When these lamps light up, they indicate in which position the switch has been set.