I have been repeatedly visiting West Harbour to track when during 2016 they'd install the planned spur extension (which would automatically
instantly bring us all-day weekend GO train service, via piggybacking off the existing seasonal Niagara train)
While they are doing good progress on the main station, the long awaited "big service increase" spur is still currently untouched -- that's the biggest-impact construction I'm most eager to see at the moment.

(Credit: me)

(Credit: me)
All possible indications is that they may not be on time for early 2016 (seasonal service start) but all limited information I have seems to point to it completing later during 2016.
One case in point, there's sewer sections currently being stored near the drains that need to be relocated. This is pre-requisite work that should ideally start before winter when the ground would freeze. The sewer sections haven't been moved yet.
Staring down from east edge of the James St N bridge over the tracks:

(Credit: me)
At the right edge of the above image, /appears/ they could be replacement storm sewer shaft segments for the planned storm drain relocation.
In several months, these haven't moved yet, which could mean construction work on this, has not yet started, to do the preliminary prep work to extend the track -- necessary to get Niagara GO/VIA trains to stop, representing a dramatic increase in service for West Harbour GO (without needing additional trainsets yet, though those will still be needed, as the Lewis layover facility completes).
Compounding all of this, is that several parties have mentioned that the John St overpass needs to be replaced by CN eventually, which also has to factor into all of this logistics.
Until this 100 meter(ish) spur is done, all we can do is
watch the Niagara GO/VIA Trains zoom by West Harbour by mere centimeters without being able to stop:
My informed speculation (aka less-than-wild guess): Late 2016, based on all available information I have, including
replies from Metrolinx, the progress on the south platform, and other information I've gleaned.
If you see any evidence of activity beyond "END OF TRACK" (the million dollar question of timing of major service increases), this is the part of construction that I'm most eagerly watching. And many others.

So snap a pic, if you see activity towards the east.