Engineering Wonders of the World
Volume I

RAILWAY SIGNALLING. 227 important is three feet or so lower ; and so on. This is made clear from Fig. 4, which shows a signal for leading from a main line along any one of four different lines through a station. It will be gathered from this that the most important is the third line from the left, and the least important is the fourth line from the left. Originally the arms would have been placed one over the other, the top arm applying to the left-hand road ; and it will be readily appreciated that to have the arm for the most important road third on the post was a bad system. Where there are several roads and junctions leading off them, it is difficult to fix the posts and so to arrange the arms that their exact relation to the lines may be clear ; but the trouble has been overcome by providing signal bridges or gantries. One of such, fixed at the south end of Crewe Station, appears in Fig. 1. This carries the signals for entering the station on four different lines, and some outgoing signals as well. Even with signal bridges, it is not always possible to get the arms within a reasonable compass, so route - indicating signals have been adopted. Only one sig- Signal i . . , , . .. . nal arm is provided, and an Indicators. . . ’ indicator, with numerals or letters, shows the direction for which the line is “set.” When the arm is at danger the indicator is obscured. The boon of these indicators may be judged from the fact that on a signal bridge at Glasgow Central Station there are only twenty-two arms instead of eighty-six. The route-indicating signal may be likened to the bell board of a hotel or a lift, where screens appear showing which bell has been rung. The indicator is illuminated from the back at night to show up the numerals. The Great Northern Railway Company were led to make a change in their type of signal arm in consequence of the Abbots Ripton collision of January 1876, due to the signals becoming unworkable through heavy snow weighing down the arms and giving a false clear signal. The central balanced arm (Fig. 5) was adopted, as snow accumulates fairly evenly on both sides of the spindle which supports the arm. Points. In the early days of railways all points were shifted by a lever placed near the switches. Then the practice grew of group- ing some of the levers together on a stage and coupling the points by rodding to the levers. There are two kinds of points—trailing and facing. Trailing points aré those that a train passes through from the rear where two lines join each other. Facing points are those which determine the course of a train when passing from one line on to either of two lines at a junction. The switches of the points are movable, and are set in one posi- tion or the other to guide the train. Their tapered ends, fitting close to the rail, remove any possibility of danger from their project- ing into the path of the wheels of the train and causing a derailment. If, however, the bar connecting the points should break, one point might be moved without affecting the other, and derailment of the next train pass- ing would be inevitable. Furthermore, there is the risk of a signalman shifting the points while a train passes over.