Not sure if this is a popular journey but why is there no rail link between Kettering and Northampton?!
Same reason there isn't one between Hitchin and Luton, it was intermediate between two rail links, Bedford-Bletchley and Market Harborough-Northampton/Rugby.
This map shows how the two interacted. Blame Beeching (and before) for removing all the cross-links between the main lines with the exception of the Marston Vale, but be glad Beeching 2 was never implimented, that would have diverted MML trains down the old Leiceter-Ruby Midland route and shut down the MML south of there.
Hitchin/Stevenage-Luton would probably load fairly well (the 100/101/102 bus routes are always very busy) but would be a nightmare to build, since it cuts through the eastern edge of the Chilterns, the whole lot in the Green Belt. Reinstating Welwyn-Luton might be more likely, but not much, whilst the link farther north between Bedford and Cambridge is probably scuppered as well.