Metro and bellows: what they have in common

1. Step 1. Metro — uses — Railway track. It runs on rails.
2. Step 2. Railway track — uses — Rolling mill. Rails are rolled.
3. Step 3. Rolling mill — is part of — Steel works. Rolling mills.
4. Step 4. Steel works — has part — Blast furnace. Blast furnaces.
5. Step 5. Blast furnace — has part — Bellows. Air was blown in by bellows, later by blowers.

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