I did some looking:

The longest underground rail tunnel in the UK is from East Finchley to Morden on the London Underground Northern line - just over 17 miles in length.



The longest underground rail tunenl in the world is:
Seikan Tunnel in Japan 33.5 miles long.
The Channel tunnel is 31.3 miles long.

Our less than 5 miles of tunnel is tiny to them. But over 4 miles of tunnel is still loooooooooong.