RAILWAY CONSTRUCTION. 319 



extends out beyond the rails, and is formed into a flat bar or 

 plate, J, sliding through the guide-holes K, K in the casting L. 

 Short upright levers, M and N, work on trunnions fixed in the 

 casting, and to M and N are attached the wires leading from the 

 signal-cabin and continuing on to the signal-posts, as shown in 

 elevation in Fig. 483. Two slots are cut in the plate J to receive 

 the curved arms of the levers M and N when they are drawn 

 downwards to pull off the corresponding signals. Neither of the 

 levers, M or N, can be drawn over unless there is a slot imme- 

 diately under the curved arm into which it can enter. When 

 there is solid plate under a curved arm, the short lever cannot 

 be pulled over, and the signal therefore remains at danger. The 

 slots in the plate J are spaced so that one will be brought into 

 position for one of the curved arms, when the points are close 

 home for the main line, and the other slot for the other curved 

 arm, when the points are set for the branch line or siding. The 

 two slots cannot be under the two curved arms at one and the 

 same time, as one of the signals corresponds to the main line and 

 the other to the branch line or siding. 



In some forms of signal-detector the transverse rod I is 

 joined on to a vertical bar which slides through guide-holes in a 

 casting something similar to the arrangement shown in the cast- 

 ing L. Longitudinal guide-holes, parallel to the line of rails, are 

 made in the casting a little above the transverse rod-bar, and 

 through the longitudinal guide-holes slide two vertical bars 

 which are attached to, and form part of, the wire connections to 

 the two signals. The wire bars have each a small tongue or 

 rectangular fin forged on to the under side of the bar, and there 

 is one corresponding channel cut in the transverse rod-bar. 

 When the switches are properly closed in one position, the 

 channel cut of transverse bar will be opposite one of the wire- 

 bar fins, and will allow one of the signals to be pulled over, but 

 the other wire bar cannot be moved. The closing of the switches 

 in the reverse position moves the channel cut so as to allow the 

 other wire bar to be pulled through, but as there is only one 

 channel cut in the transverse bar, only one signal can be pulled 

 over for each position of the switches. 



Throw-off or trap points, are introduced to throw an engine 

 or train off the rails of a siding on to the ballast, and so avoid a 

 collision with any other train which may be standing or passing 

 on the line of rails with which such siding forms a connection. 



