I did some digging on the matter and managed to procure a February 1988 copy of Railway Age that had an article about the matter (FEC: System-Wide with ATC).
At least as of the original installation, the FEC system used a carrier signal of 40 Hz, and their installation of the Harmon signal system was known as Ultra-Cab 40.
Speed control was accomplished by comparing a braking curve to the actual speed and determining and a "time to application" before penalty braking would be applied. I'm not an expert about train signalling systems, but that sounds a lot like modern PTC.
There are more details in the article itself, and I wish I had a better understanding of the actual subject matter so I could convey them to you better. I legally can't upload the file, but I can use it to try and answer any questions you might have.
At least as of the original installation, the FEC system used a carrier signal of 40 Hz, and their installation of the Harmon signal system was known as Ultra-Cab 40.
Speed control was accomplished by comparing a braking curve to the actual speed and determining and a "time to application" before penalty braking would be applied. I'm not an expert about train signalling systems, but that sounds a lot like modern PTC.
There are more details in the article itself, and I wish I had a better understanding of the actual subject matter so I could convey them to you better. I legally can't upload the file, but I can use it to try and answer any questions you might have.