Historical version 6 of East London Line (view current version)
It goes through Brunel's 1840s Thames Tunnel, which is the oldest extant tunnel under a navigable river. It's due for extension both northwards (over the old Broad Street-Dalston viaduct) and southwards (possibly sharing Railtrack lines towards Wimbledon and Croydon).
{Todo: CLUG, rolling stock and thetube links}