Radio navigation and railway semaphore: what they have in common

1. Step 1. Radio navigation — uses — Radio. Radio beacons.
2. Step 2. Radio — uses — Morse code. The first radio transmissions were in Morse code.
3. Step 3. Morse code — is used in — Electrical telegraph. Morse encoding.
4. Step 4. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.
5. Step 5. Optical telegraph — evolved into — Railway semaphore. The semaphore idea.

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