A three-part series on the fiber plant beneath modern railroad signaling: where the cable physically runs, how the network on it talks, and who keeps the record true. One corridor, carried through all three parts.
The physical plant: why signaling went to glass, what lives inside a 48-strand cable, how a midspan access works without touching the express path, and how a loss budget quietly fails at the far end of a fifty-mile corridor.
The logical plant: which devices may share a wire with vital equipment, an addressing schema that encodes the corridor into four octets, and the race between ring reconvergence and a vital processor's stale-data timer.
The institutional plant: why the grant that built the corridor funds nothing that keeps its records accurate, how an OTDR baseline doubles as a defensible condition assessment, and why change control is the only difference between a record and a rumor.
The Union Switch & Signal master/slave polling protocol still moving non-vital code-line traffic across North American railroads: frame format, byte-stuffing, CRC-16, and the checkback control handshake, reconstructed from the vendor manuals.
For over a century the vital safety logic that keeps trains apart has been built from relays. How the signal relay works, why gravity makes it fail-safe, and how contact chains become interlocking logic that modern vital microprocessors still mirror.