Im wondering if it would be possible to run the existing tfl rail service into liverpool street low level, terminate and turn them there this would allow the liverpool street platform works to be completed and nine cars on the great eastern branch.
I recall reading this was examined as a potential option during the lead up to the original opening date when it became clear the core wasn't ready, and was dismissed for a bunch of reasons, including where the CBTC boundaries are, issues with passenger flow and volumes (it's not expected everyone will continue to get off Shenfield trains at Liverpool Street), location and capacity of crossovers that could be used to reverse the trains, interference with completing work at Whitechapel and testing other services and the Abbey Wood branch, emergency evacuation problems, etc. etc.
Basically a nice idea on the surface but impractical once you scratch it.