Looking At the Thames Tunnel
The Thames Tunnel is the oldest underwater tunnel in the world and was built between 1825 and 1843 by Marc Brunel and his more famous son Isambard. It is now used to carry the East London line under the Thames and you can actually look into the tunnel from the platforms at Wapping station.
I was looking from the Northbound platform, just by the exit and the stairs that lead up to the street. When the station was designed, they decided to put protective railing to stop you failing on the line, but these do not obscure the view down the tunnel as the train approaches.