More than likely it used Automatic Block Signaling, invented in the late 19th century. Train detection is done by looking for the electrical leakage across the

More than likely it used Automatic Block Signaling, invented in the late 19th century. Train detection is done by looking for the electrical leakage across the rails through the steel wheels & axles of the train. https://en.wikipedia.org/wiki/Automatic_block_signaling https://en.wikipedia.org/wiki/Automatic_block_signaling It can't handle as many trains as centralized control, but it has the advantage of not needing a network in remote areas - just buried wires. It's the equivalent to relay logic. (I finally get to put some of my teenage model railroading knowledge to work)