Isn't this a bit of a chicken-and-egg though? Traffic volumes on the A421 (and loadings on the X5) suggest there's an awful lot of people moving along that corridor, so providing a decent rail service between MK (as in Central MK) and Bedford could make a huge difference to passenger numbers, I'd imagine, even before the key link to Cambridge is finally built. Fast Oxford-Bicester-Bletchley-Bedford with a coach link (maybe a vestige of the X5, maybe better just a new link with fewer stops east of Bedford) from Bedford station to Cambridge station would surely also make a big difference. It might even compete with rail via London for the Oxford-Cambridge trip.
To add a bit of data, Google Maps tells me that starting right now (as a random datum) it takes 58min to get from Milton Keynes Central station to Bedford station at the moment, … using the X5, and I depart at 1024 to get there at 1122 (so I arrive 1h8m from now). By rail, changing at Bletchley, it takes 1h23min, departing at 1042 and arriving at 1205 (so I arrive 1h50m from now). If I jump in my car and set off it takes 28m via the A422 and I arrive at 1043. Get the rail journey time down to 45min and you'd take a lot of those X5 passengers,
and I'd hope at least some of the A421 (and A422) users, surely. But someone will have been doing these calculations (surely
…
(Oh yes, and it's 28.7km by bike, which Google Maps thinks would take 1h30m (sounds about right I guess, much as I'd like to think I could do it quicker
, so I'd be quicker by cycling than by taking the train, setting off at 1015! Remaining alive for the duration of the journey might be a big ask though, unless the A422 has a cycle track alongside it, which I doubt. Interesting thought experiment to cost one of those with marble-smooth tarmac, priority at junctions, etc., for maximal cycling speed, vs an upgrade to the Marston Vale line, though).
(Final aside, it's currently only about 30 minutes slower to cycle the 65km from my house in Vale of White Horse to my parents' in Central Bedfordshire, than to take public transport (of course generally I drive, taking an hour, vs 3h at great expense using two buses and two trains (via Coventry!!!) or 3h30m by bike with extra snack costs!) … quite looking forward to the opening of the western section, from that perspective.)