Nebulizer and tunnelling shield: what they have in common

1. Step 1. Nebulizer — has part — Compressor. A compressor.
2. Step 2. Compressor — is part of — Refrigerator car. A refrigeration unit.
3. Step 3. Refrigerator car — has part — Bogie. Bogies.
4. Step 4. Bogie — is part of — Electric locomotive. Bogies.
5. Step 5. Electric locomotive — is used in — Metro. The first lines were steam-operated; electric trains since 1890.
6. Step 6. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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