RAILWAY CONSTRUCTION. 315 



With a modern interlocking frame, and assuming the normal 

 position of all the signals to be at danger, then before a signal 

 can be lowered for an approaching engine or train all the 

 switches and corresponding signals, from any lines or sidings 

 connecting with the line to be signalled clear must first be set so 

 as to prevent any engine or train coming out of such connecting 

 lines or switches on to the line to be made clear. In a similar 

 manner, before the points and signals can be set to permit an 

 engine or train to pass from a siding on to the main line all the 

 necessary signals must first be set to danger to prevent the 

 approach in either direction of any engine or train on the main 

 line about to be occupied. The mechanical arrangements of the 

 interlocking frame are so exact and complete as to effectually 

 prevent any but the proper combination being made. An 

 untrained or inexperienced signalman might inadvertently 

 attempt to pull over a wrong lever, only to find it securely 

 locked and immovable under the control of other levers. The 

 proper sequence of levers must be made, and the accurately 

 adjusted mechanism automatically prevents mistakes which 

 formerly occurred with the old hand- worked signals from the 

 oversight or confusion of the signalman. 



The interlocked switches or points are worked from the 

 signal-cabin by light wrought-iron tubing (termed rodding) or 

 channel-shaped iron bars supported on fixed iron rollers, and the 

 signals by galvanized wires running over light pulleys. Modern 

 signals are always weighted at the signal-post, so that in the 

 event of the breaking of the pulling-wire they will fly back to 

 their normal position of danger. 



The facility and precision secured by the interlocking 

 machinery enabled other valuable accessories to be introduced 

 for the more complete signalling and protection of train-work- 

 ing. Amongst these may be mentioned the facing-point bolt- 

 lock and rocking-bar, signal-detectors at points, and throw-off or 

 trap points. 



With the old-fashioned hand-worked switches the man stand- 

 ing alongside could see whether the sliding-rails were properly 

 closed, and also when the last vehicle of the train had passed over 

 them ; but when important main-line-facing switches or points 

 are worked by rodding from a signal-cabin some distance away, 

 it is necessary to have some reliable means to ensure that the 

 sliding-rails are actually brought close home, and also to prevent 



