To explain this, it helps to understand platform numbering.
On all train stations in Sydney, platform 1 is always on the left hand side of the station when facing in the direction of Central Station. In addition, trains always travel on the left track, in the same way cars always drive on the left hand side of the road. As a result, trains on platform 1 always go to Central; while at stations with 2 platforms, trains on platform 2 always come from Central. Tracks with trains going into Central are called up tracks, while tracks with trains coming from Central are called down tracks.
(In some cases it gets a bit complicated. For example, the direction of Central on the City Circle is a bit tricky, and based on starting at Central itself and going counter-clockwise to Town Hall via Museum. Also, when there are multiple pairs of tracks the direction of trains either alternates: up/down/up/down/etc, or get paired up: up/up/down/down.)
Platform 1 at Wynyard Station
Wynyard station has platforms 3 & 4 (North Shore/Northern/Western Line trains) and platforms 5 & 6 (City Circle trains), but is missing platforms 1 & 2. It did use to have platforms 1 & 2 for trams that crossed the Harbour Bridge and then terminated at Wynyard in the CBD. These platforms were reserved for a future rail line to the Northern Beaches, using the Eastern 2 lanes on the Bridge and 2 of the 4 platforms at North Sydney.
Today it has been turned into a car park, but you can see the tram/train style design of the tunnels. The lanes on the Harbour Bridge are now used by cars, though one of the two was converted into a bus lane when the Harbour Tunnel opened about 20 years ago.
Platform 1 at St Leonards Station
The original St Leonards Station was built in 1890 on its current location North of the Pacific Highway. However, in 1989 it was relocated to a temporary station South of the Pacific Highway so that a new station could be constructed. This was completed in the year 2000.
Meanwhile, in 1998, the government announced it would build the Parramatta Rail Link. This would start at Parramatta, connect up to the Carlingford Line, then travel through a tunnel to Chatswood via Epping, before continuing South to St Leonards. Part of this was an amplification of the track between St Leonards and Chatswood, from 2 tracks to 4, in order to allow trains from Parramatta to terminate at St Leonards and turn around to go back to Parramatta without disrupting trains on the North Shore Line.
However, budgetary constraints meant that the Epping to Parramatta portion of the line was cancelled in 2003, as was the quadruplication of the Chatswood to St Leonards portion. But the new St Leonards Station had opened 3 years earlier, to the specification of and with the expectation that it would be a 4 platform station, not just a 2 platform station. Hence, while only the inner 2 platforms (2 and 3) have tracks on them, the 2 outer platforms (1 and 4) have no tracks and a fence to keep people out.
Today the Chatswood to St Leonards quadruplication is set to form the start of the Second Harbour Rail Crossing (and the only part that will be above ground), a new rail line connecting Redfern to Chatswood. It will then connect to the North West Rail Link on the Northern end, and the Bankstown and Illawarra Lines on the Southern end (but only through to Cabramatta/Lidcombe and Hurstville respectively).