Electrical features of block signaling

It has been only within the last decade that electrical science has played any important part in block-signaling. Previous to that time the only use for electrical apparatus in signaling was in the form of a few batteries for the track circuit, and for the operation of pin-valve magnets in the electropneumatic system of signaling and interlocking. But in the past few years electricity has come steadily into use as a means of transmitting the operating power, until at the present time it is the prime element in signaling.