I'm guessing what ever the solution Railcorp have more than likely done their homework and are looking for a low maintenance solution that also reduces vibration and noise on the bridge, perhaps even spreads the weight more.
This 'solution' is quite common. I'm pretty sure one of the two Cooks River bridges at Wolli Creek got this treatment a few years ago - wooden bridge-timbers (super sized sleepers) with the rails attached in the usual timber sleeper way were replaced with pre-fab concrete panels that had pre-fitted resilient rail mounts fitted. The rails were lifted, the bridge-timbers removed, the steel structure cleaned, patched where required and painted - then the concrete panels were lifted on, aligned and bolted down to the steel superstructure of the bridge and then the rails refitted. It didn't take all that long either.
The concrete is probably a special type too - I've seen building slabs where the aggregate is plastic balls, not stone to make the resulting slab lighter. Not saying they are using this here, but it's a possibility to reduce the 'static load'.
The main enabler for this sort of conversion is the adjustable resilient rail mounting hardware available now - adjustable and vibration absorbing.