I rode the new stretch of cycle path the Thursday after Christmas, on the way home from my first trip to Glentress for many a year. I agree that the surface is very good - although I did notice that the edges are a bit wavy in places! I don't know whether there's more to be done along the edges - soakaway drainage or something?
I also explored the "stubs" at the Shawfair end a bit. The first one on the south side of Shawfair Avenue seems to follow the continuation of the route of the old railway line, and heads off across a rough field toward what should probably end up as a junction with The Kaims, as previously mentioned on this thread. Further along, just short of the A6106 roundabout, is the stub which is the start of The Kaims itself. On the north side there was one 'blind' stub that seemed to go nowhere in particular (yet), and a second one further west that currently seems to exist purely to provide an access route to the old Newton church - that area is otherwise surrounded by preparatory earthworks (and what looks a bit like a miniature, semi-subterranean velodrome, but which is probably destined just to be a pedestrian path around a water feature of some kind).
I went further on, in to Newton village itself, and crossed the Borders Railway via a seemingly interminable series of zig-zag ramps. Then on past Shawfair station, and looping back to the A6106 past what looked like yet another building site, thence back to the cycle path and onwards back the way I'd come.
@chdot: The Marshalling yard is no more.
My understanding is that on the 'up' (eastern) side of the site a new depot for the EGIP trains has just recently been completed, and I've heard talk of a facility going in there for the new Hitachi trains in the ECML as well. Network Rail had a change of policy a couple of years back which meant that the PW depot, which was on the verge of becoming moribund, is now an infrastructure hub dispatching engineering trains across the whole of Scotland. I've also heard that the line through Millerhill will at some point become available as a diversion route for passenger services off the ECML. So it doesn't look like the place is going to cease being a railway altogether for a while yet, and it may actually get busier than it's been for a while before very long.
I'd agree that it should not be (?have been?) beyond the wit of man to provide a link to the access road for the Energy Centre/RERC Project site on the 'down' (eastern) side, and thence onwards to Whitehill Road and New Craighall. (I don't know whether there might be issues with ground contamination in the area where Monktonhall Colliery used to be.)
Another possibility might be to put in another zig-zag ramp bridge over the railway where it severs Whitehill Road just north of Old Craighall Road.
Overall, though, it looks to me as if there's so much potentially going to be built in that whole area - going by the various stub roads and cleared sites - that there will likely be some reluctance to put in new facilities until the plans are a good deal clearer. The key, in that case, would seem to be to make sure that active travel is properly accounted for in those plans.