I linked a page describing the construction system in the earlier EWR thread, but here it is again (scroll down to "Modular Abutments"):
Listen to this article Build, build, build! – heartening words for construction. Better, greener, faster – the imperatives for the industry. But what remains unspoken is the poor past-performance of the sector. Since the mid-1990s, the productivity of manufacturing has improved by 50 per cent...
www.railengineer.co.uk
Essentially the precast "boxes" are permanent formwork for insitu concrete poured within them.
I agree that the amount of concrete and the numbers of beams, seems excessive. I understand the wish to avoid piers between WCML tracks, but had I been designing the structure, I'd have tried to utilise portal frames at (say) 10m centres, with the deck beams parallel to the EWR tracks and spanning between the portals, forming a deck about 8m wide for the two tracks. The portals would have been substantial, but I think overall the volume of concrete, and certainly the number of beams, would have been reduced. Maybe the designers tried that and it didn't work, or maybe not.
(One of my roles in the last decade before I retired was carrying out buildability checks on civil engineering designs produced by young engineers within my organisation. Too often engineers with little site experience, and no background in designing by hand calculation, took output from their software as being the only possible answer, without questioning whether it was the most appropriate solution, whether appropriate input parameters had been used, or even if it was practical to construct. Fortunately I managed to stop some horrors before they went to tender.)