Tunnel portal and heat pump: what they have in common

1. Step 1. Tunnel portal — is part of — Tunnel. Tunnel portals.
2. Step 2. Tunnel — uses — Rock drill. Driving the Mont Cenis tunnel.
3. Step 3. Rock drill — uses — Compressed air. Pneumatics.
4. Step 4. Compressed air — is used in — Air brake. The brake is controlled by air pressure in the train pipe.
5. Step 5. Air brake — has part — Compressor. A compressor pumps the air.
6. Step 6. Compressor — is part of — Heat pump. A compressor.

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