The Invention of the Track Circuit / The history of Dr. William Robinson's invention of the track circuit, the fundamental unit which made possible our present automatic block signaling and interlocking systems

by American Railway Association

The following sections are from this circular of January, 1874:

"When it is desired to operate a secondary signal thrown forward or back of the primary, a line wire H is used, attached to the primary signal C in such a way that the secondary signal cannot possibly operate unless the primary signal C is first exposed, thus closing circuit on the wire H. The primary signal battery K is used to operate the secondary signal."

"To set the signal from an intermediate station a wire from each rail of the section A is run into the station. When these wires are connected by a key, the current from the battery I is placed on short circuit, and the signal exposed as before." (See Fig. 7.)

"The following functions may be embraced in the signals of a single section. BLOCK SIGNALING, both automatic and manipulated, SWITCH, DRAWBRIDGE, ROAD-CROSSING, and STATION-APPROACH SIGNALING, and BROKEN RAIL DETECTING."

"In this system it will be observed that, since the signal is exposed mechanically, any tampering with the rails or connections, or failure of the battery, will invariably result in exposing the signal; any error therefore which may occur from any cause will be in behalf of safety. It is impossible to show safety when the danger exists which the signal is designed to avert."

During the early seventies Mr. Robinson made other closed rail circuit installations on the Philadelphia & Erie and other railroads in Pennsylvania and Maryland.

Visit of the Pennsylvania R.R. Officials

On October 24, 1873, a special inspection train of the Pennsylvania railroad passed over the Philadelphia & Erie railroad, westward. The Pennsylvania R.R. officials aboard were: Mr. A. J. Cassatt, at that time general manager; Mr. Gardner, general superintendent; Mr. Lewis, controller; Mr. Robert Pitcairn, superintendent western division, and Mr. Frank Thomson, superintendent motive power.

Mr. Wm. A. Baldwin, general superintendent of the P. & E. road, was of the party, and Mr. Robinson joined the party on the latter road, and continued with it through to Erie, which was reached in the evening.

Stops were made at Ridgway on the Middle division and at Irvineton on the Western division to examine the Robinson closed circuit rail system of signals, which were in full operation at those points. A thorough examination and various tests were made, to all of which the signals responded promptly and perfectly.