Great Western Coffee Shop

I'm curious as to why the new section of track to Heathrow uses 4 rails rather than the normal 2 per track.

Pictures http://www.getwestlondon.co.uk/news/local-news/first-crossrail-tracks-laid-heathrow-8162527

To retain the bogies on the track round tightish corners?

Looks very strange. Like they have laid a narrow guage line inside the standard gauge track. The fairly even spacing of the four rails doesn't look like the thing that is sometimes done on curves.

Aren't the inner rails merely conveniently stored pending their installation either side of the bridge work? i.e., they'll be dragged to their new position when ready.
Just studied the pictures closer and can now see that they are fixed to the sleepers.  They look like derailment containment on the bridge section.

I went over the repaired and relayed track over Hayle viaduct today. It also has two rails down the centre of the 4 foot to ensure a possible derailment is kept on the viaduct. There are also curved rails at the side on the approach for the same reason.