Not a train but definitely tunnel nerdery.
If you go to the Salmon and Ball pub at the junction of Bethnal Green Road and Cambridge Heath Road. There is a another tunnel accessible through a wrought iron gate at the back of the cellar which run west via Oxford Street and East to no idea where.
Don't have a clue what the tunnel is for
In Bloomsbury, all of University College London's buildings are connected by underground tunnels. You can go all the way from the School of Tropical Medecine via the main building and on to the Geography department without seeing daylight.