Tunnel lining segments and railway semaphore: what they have in common

1. Step 1. Tunnel lining segments — is part of — Tunnelling shield. Lining segments installed behind the shield.
2. Step 2. Tunnelling shield — is used in — Tunnel. Shield tunnelling.
3. Step 3. Tunnel — has part — Street light. Lighting.
4. Step 4. Street light — has part — LED. LEDs.
5. Step 5. LED — is part of — Traffic light. LEDs.
6. Step 6. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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