just to say this was on the Sydenham Society's radar from at least 2006. Before the formation of the FH Soc, the Syd Soc used to look after Forest Hill's interests too. There was an initial agreement between the Syd Soc and FH Soc that because the subway issue was getting quite protracted, that the Syd Soc would continue its campaign to its conclusion. (I have no idea what the sitaution is today)
So representations about the subway have been made to the rail operator and the Council over many years, but with very slow progression in terms of what works need doing and who was going to take (financial)responsibility.

Keep an eye on the Sydenham Society's website in case they have any more information
http://www.sydenhamsociety.com/