Haverhill is an interesting one.
Comparison with Corby is irrelevant. Kettering/Corby is a low growth area with no intermediate stations. Haverhill links with Cambridge, which is booming, and has lots of other residential, business and educational traffic generators along the way. These include Linton, the Abingdons, Babraham, Sawston, Stapleford and Shelford. It is also right side of Cambridge for the rapidly expanding Biomedical Campus including 5 hospitals, 3 existing and 2 more planned, and the new Cambridge South station.
Let's look at existing bus provision. Cambridge-Haverhill has a double decker every 30 minutes and they are very busy, with extra services at peak times. And still people are turned away at Addenbrookes in the evening, so that one service has to start there to clear the queue. Linton is getting increasing problems with on street parking by commuters getting the bus to/from Cambridge. Sawston, Stapleford and Shelford have a separate service, also double deckers, which runs every 20 minutes. And that's with a very unreliable bus service: punctuality is dire because of traffic congestion in Cambridge and cancellations are also a problem because of driver shortages.
A Cambridge-Haverhill service with stops at Cambridge South, Shelford, Sawston, Babraham, Great Abingdon and Linton would almost certainly exceed the 300k journeys per year figure quoted for Corby, especially if more housing is built. The existing bus services are already achieving that level of traffic.
But there are significant obstacles that would make reinstating the railway very expensive, especially reinstating the junction at Shelford and crossing the A11/A505. Without EWR Haverhill can't happen, because there are no train paths to/from Cambridge. With EWR, and 4 tracks from Shepreth Branch Junction, that would not be a constraint, but it is still most likely that some sort of busway would deliver some of the benefits at much lower cost. The big disadvantage with a busway is that it has to go round the eastern edge of Stapleford and Shelford, which is very unpopular, and considerably less useful than stopping at the existing rail station.