Northern line

The  was a line on the London Underground.

According to Clara Oswald, it was full of unpleasant smells in summer. (COMIC: The Swords of Kali)

Liz Shaw took the northern line from Hawthorne when returning to P.R.O.B.E. headquarters. She remarked to Lou Bayliss that her lateness was due to the "bloody northern line". (HOMEVID: The Zero Imperative)