Four-wheel drive and tunnelling shield: what they have in common

1. Step 1. Four-wheel drive — uses — Axle. Four-wheel drive sends engine power to both axles, front and rear.
2. Step 2. Axle — is part of — Wheelset. A common axle.
3. Step 3. Wheelset — is part of — Bogie. The bogie carries the wheelsets.
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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