Combustion and railway semaphore: what they have in common

1. Step 1. Combustion — is used in — Gas lamp. The flame gives the light.
2. Step 2. Gas lamp — uses — Coal gas. Coal gas.
3. Step 3. Coal gas — is used in — Traffic light. A gas lamp lit the signal.
4. Step 4. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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