This is socio-economics. Every minute counts to every individual. The value of each minute varies by individual, so in transport assessment an average value is taken.
Northampton had over 3m passengers per year pre Covid, and is probably in the region of 2.5m now. At a guess, 80% - 2m - of them are heading south. That’s 8m minutes a year lost (collectively) by those travellers. Add in those travelling that way from Long Buckby, Rugby, and other places and 10m minutes is a reasonable estimate.
Assuming a value of time of £10/hr, (values in the TAG data book vary between £4.54 and £29.18 depending on the reason for travelling), that’s a loss to society of £1.67m a year. This would be offset by the gain in journey time by those choosing to use the new station. But it would be very, very unlikely to generate sufficient traffic with Sufficient benefit to make it net worthwhile: for example…
Castlethorpe has a population of just over 1,000, add in Hanslope and Hartwell and it’s about 4,000. Say 200 of these people will use the train frequently, generating 60,000 rail trips a year (an optimistic estimate), mostly driving to MK. If they all switched to Castlethorpe, these people would would save perhaps 10 minutes each on average - collectively 600k mins. Then assume a similar number of people transfer from other modes to rail, ie generated rail traffic, and also save 10 minutes each on average, ie another 600k minutes. That’s a total of 1.2m minutes saved pa. Now let’s say there’s lots of housebuilding and the population doubles; that’s 2.4m minutes saved. That is still only a quarter of the minutes lost by those already travelling, ie society is 7.6m minutes worse off.
Socio-economic assessments are much more complex than this in real assessments, but I hope this explains in simple terms.
