Shafts for harnessing a horse and rolling mill: what they have in common

1. Step 1. Shafts for harnessing a horse — is part of — Sledge. A rod.
2. Step 2. Sledge — evolved into — Cart. The wheeled cart replaced drag sledges and sleds.
3. Step 3. Cart — is part of — Wagonway. Wagons rolled on the rails.
4. Step 4. Wagonway — evolved into — Railway track. Cast-iron and steel rails replaced wooden ones.
5. Step 5. Railway track — uses — Rolling mill. Rails are rolled.

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