"A tower operator of many years experience that worked for NYC/PC/CR in the Detroit area tells me that there were several train annunciators located on the Detroit to Jackson and Detroit to Toledo branches. Some of them were located around yards and stations where trains would set for a while, while they made setouts and pickups. They would beep out in Morse code letters such as that used for towers, like "FN" in Trenton. They could be turned off and back on by the train dispatcher via the same selector-set buttons that were used to ring the operators in the towers. Of course, if a train dispatcher turned one of those off, he'd have to remember to turn them back on again, or he wouldn't get an OS from the next train to pass there! We called these annunciators "OSs".
It was really interesting to see how these worked; a signal maintainer showed me one of these. A train passing over a short track circuit section would actuate a small sealed unit located in a wooden booth. The sealed unit contained a miniature cam shaft, which would rotate. The cams on the shaft depressed small buttons that "beeped" out the code.
The Bay City and Mackinaw branches had open-air mics for annunciators."
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