Which is the oldest underground line?

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Tags

1. #lines
2. #metropolitan
3. #circle
4. #hammersmith
5. #between
6. #bishop
7. #farringdon
8. #section
9. #newest
10. #opened
11. #street
12. #railway
13. #paddington
14. #running
15. #first
16. #underground
17. #oldest
18. #known
19. #london
20. #world