In this connection a few historical notes on the origin and introduction of the closed rail circuit system of automatic electric block signaling on railroads may prove of interest.
In 1870 Mr. William Robinson exhibited at the American Institute Fair held in New York City, an elaborate working model of an automatic electric signal system for railroads. This was a road crossing signal operated by trains approaching in either direction. When at a suitable distance the train set a gong ringing at the road crossing ahead, which continued sounding an alarm until the train had passed, when it ceased ringing. In this model the relays were de-energized by short circuiting, although the signal was operated on the normally open circuit plan. This is believed to be the first case in which short-circuiting had been used in the operation of railway signals.
In 1871 Mr. Robinson installed this system as an automatic block signal on a block over a mile in length, at Kinzua, Pa., on the Philadelphia and Erie Railroad. This installation embodied a relay, a large visual signal under control of the relay, a heavy electric gong operated in conjunction with the visual signal, all at the signal station. From this station an overlap extended to the agent's station a mile ahead. Here a signal bell was provided so that when the visual signal was actually in the danger position it closed circuit on the bell magnet in the agent's station, the hammer remaining against the bell until the reversal of the distant signal opened the circuit of this check signal.
This system worked perfectly, performing all claimed for it; but it was a normally open circuit system, the only principle ever dreamed of up to that time for operating an automatic electric railway signal.
Immediately on the completion of this open circuit installation Mr. Robinson began to look for weak points about it, and soon discovered several now well known as inherent in all normally open circuit systems, not the least of which was that if the circuit were broken or the current failed from any cause the signal would remain at safety, thus showing a false signal although danger might be imminent, a radical error in principle fatal to the reliability of any normally open circuit system of signaling.
He therefore, after much study, devised the closed track circuit system, the construction and operation of which are clearly described above by the Interstate Commerce Commission.
In devising this system Mr. Robinson reasoned that to make an efficient and reliable system every pair of wheels in the train must control the signal, whereby a single car on the block, or a break in any part of the circuit, or loss of current from any cause affecting the relay, would keep the signal at danger as effectively as the presence of a whole train on the block.
These considerations led him to the invention of the closed track circuit operating as heretofore clearly described by the Interstate Commerce Commission.
Before making tests of the system, however, he applied for and was allowed basic patents on the closed track circuit system in the United States and France, the United States patent dated August 20, 1872, No. 130,661, and the French patent February 29, 1872, No. 94,393.
Having all the signal apparatus in operation at Kinzua, in the open circuit system, as above described, it was a simple matter for him to test the closed circuit system at this point. He therefore divided the opposite rails of the track into sections insulated from the adjacent continuous track rails and connected the relay terminals to these sections at one end and similarly connected a battery thereto at a suitable distance from the relay, thus forming a closed track circuit.