The blue barrier in the wall blocks access to steps which lead to the northern end of the Greenway.
The Greenway is a pedestrian path, urban wildlife and conservation area which runs through the New England Quarter. The section nearest Prestonville hasn’t been completed yet but you can now see the access point on New England Road between the two railway bridges. The entrance leads to steps that take you up and over the railway bridge and on to a path that was the original 1841 London-Brighton line.
According to Cityparks, the Greenway won’t be open to the public until the planting scheme has become more established, and the adjoining developments are complete.
Prestonville 
I asked somebody in the planning department about this a while back. She reckoned it wouldn’t open till late summer or autumn.
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It’s still not open a year later – yet you can see it from the new development near the station – what’s going on and why so long as it seems to be there?!
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I’ll see if I can find out from Cityparks
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