Mechanical and Electric Interlocking.

No wide difference in principle exists between the mechanical and electric systems of yard control. Each has its proper place in view of the facilities desired. In actual practice the essential difference in system consists of the “track circuiting” of the main running lines in electric “interlockings” preventing a train receiving a clear signal into an occupied track. In many instances a combination of mechanical and electric operation provides the most flexible method of working.

The main factors suggesting an electric installation are:—

1.	An available power supply.
2.	To obtain concentration of control. (One electric machine can operate the longest yard whereas mechanical operation of points is limited to approximately 250 yards.)
3.	Small stations, where its installation can enable the clerical staff to operate the signalling, as no heavy manual work is then required.
4.	Very large installations, where the reduction in the number of levers required for electric interlocking permits one man to operate the yard.
5.	The advantage of track circuiting of main running lines to provide protection for standing trains, thus supplementing the human factor.
6.	The assistance to enginemen in speed control. Electric or partial electric interlocking facilitates the use of Three Position (speed) signalling.
7.	The benefit of an illuminated diagram to provide visual indication of yard conditions to the signalman.

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With mechanical interlocking alone, the onus of ascertaining that a route is clear before signalling a train in rests with the signalman. With combined electric and mechanical interlocking, in addition to the usual mechanical safeguards, the track circuiting of the main running lines governs the signals giving access to those lines, so that when a track is occupied its signal lever (if a mechanical signal) is locked in the normal or danger position. With colour-light or power-worked signals, an occupied track automatically keeps its entering signal at danger, whatever the position of the lever.

Electric interlocking considerably modifies but does not eliminate the mechanical locking governing the various combinations of levers which may be pulled over together. It supplements the mechanical locking by providing additional locks on the levers and a further check on the signal indications where such is made necessary through the track being occupied, the permanent way damaged, or faulty adjustment, etc.