Yes, that's the viaduct I was referring to (apologies for not replying sooner!). AIUI the curvature is too severe for modern heavy-rail use, so as much as it could be useful otherwise, it's a non-starter.
My concern with the Camden proposal is that supposedly it could be returned to railway use even after conversion... but once it's established as a recreational space, the howling from the more extreme environmentalists would be measurable on the Richter scale. See also some of the Sustrans cycle routes: they say they'll hand them back if needed, but when it comes to the crunch they'll put up a fight.