In August 2014 without warning the Triangle (the area of Walthamstow Marsh in the middle of the three railway lines), was closed. Both entrances were blocked. It had become increasingly inaccessible in recent years, partly because it was overgrown by brambles and partly because it was being used as a place for sexual pick ups, primarily (though not exclusively) by gay men. Whoever closed off the area (possibly Network Rail after tipping off LVRPA) did so without bothering to tell users what was going on. See Triangle in happier days . There is a suspicion that this may be related to Crossrail since it's unclear exactly where Crossrail 2 (if it ever gets the go ahead) will connect with the existing railway lines. However, Crossrail 2 is probably years and possibly a generation away from actually being built, so if Crossrail is the motivation, it seems utterly absurd.

If, on the other hand, the idea is to stop homosexual activity, it's probably illegal. Being gay is not a crime and so long as LVRPA byelaws are complied with, clamping down is clearly discrimination. It can hardly be said that the idea was to let people who are not gay use the area, since the closure has prevented everyone getting access!
Network Rail's case, such as it is, has been forwarded by Walthamstow MP Stella Creasy, who took up the case on behalf of Claire Weiss and other constituents. This link shows the email sent to Stella and then forwarded to Claire .  
Katy Andrews says it's a mystery how the Triangle has remained designated as operational railway land since in the 1990s London Borough of Hackney took ownership of the area, as the largest of six parcels of land given to Hackney by the Highways Agency (bought from the National Grid and British Waterways in Hackney and from Railtrack in Waltham Forest) in exchange for the loss of part of Wick and Arena Fields, which are and were Registered Common Land, to the M11 Link Road (now the A12). Although the land is clearly owned by Hackney it is actually in High Street ward of London Borough of Waltham Forest.

Detail from John Nash/David Gibbins (1981)'s map.
Lea Marshes