Crescent Wood Tunnel (aka Upper Sydenham Tunnel)
This was on the Nunhead-Crystal Palace branch of the London Chatham & Dover Railway Opened: 1865, Closed: 1954
The other end is just off Wells Park Rd N51 25.889 W0 04.216
The tunnel portal is reached down a steep path. Could be muddy and slippery in wet weather.
The South portal is adjacent to the site of the old Upper Sydenham station, described in: Subterrania Britannica
A couple of other interesting sites are:
AbandonedStations.org.uk
London Footprints