I honestly can't see it happening past Bedford. I expect that EWR will connect with a bus service to Cambridge via the guided busway and of course, the Black Cat roundabout!
Seriously, where, outside of London and in England would they build a new railway, re-configure an entire mainline station including roads and parking and build a new double-track permanent way for about 30 miles with connections to all mainlines it crosses?
It's a shame but it just seems beyond the will of all authorities to collaborate and build an integrated transport solution.
So yes, a nice guided bus will fill the central section, I guarantee it (well not entirely)