Cast iron lamp post and steam locomotive: what they have in common

1. Step 1. Cast iron lamp post — is part of — Gas lamp. A cast-iron post.
2. Step 2. Gas lamp — evolved into — Street light. Gas lamps.
3. Step 3. Street light — is part of — Tunnel. Lighting.
4. Step 4. Tunnel — is used in — Metro. Tunnels.
5. Step 5. Metro — uses — Railway track. It runs on rails.
6. Step 6. Railway track — is used in — Steam locomotive. It runs on rails.

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