Why is London Underground so hot?

Related questions

1. What time Metro opens?
2. Why is the Bakerloo line called that?
3. Can you drink tap water Russia?
4. How much is a 7 day unlimited MetroCard?
5. Which Tube line is deepest?
6. Is the Metro free?
7. How do I get from Dubai airport to the city?
8. Which London Tube line has the most stations?
9. How fast does the central line go?
10. Which London Underground line opened first?

Tags

1. #advertised
2. #place
3. #small
4. #tunnels
5. #passengers
6. #temperatures
7. #trains
8. #station
9. #around
10. #equipment
11. #amount
12. #underground
13. #warmed
14. #coming
15. #increased
16. #largely
17. #slowly
18. #generated
19. #early