Picking up on those 2 towns, St Neots is now the largest town in Cambridgeshire and set to grow even bigger with a further 3700 houses planned for East of the ECML (next to Loves farm) Population is over the 40,000 mark and set to rise to something like 60,000. Camborne is approaching 10,000.
When the B&C closed Sandy pop was 5,000 but is now 13,000, it is unlikely to have any major further expansion as it's constrained by the A1,ECML & River, Biggleswade is being expanded (Kings Reach) and is 18,000 with 25,000-30,000 planned.
With the straight line strategy it ignores the topography of a large escarpment (Everton) in the way which is why the original B&C went via Potton to skirt around it. There's also the issue of no towns or population to actually use it.