EMR Intercity services are, I believe, making additional calls at Wellingborough, Kettering and Corby (which is crying out for a second platform). Everything northbound calling at Corby has to switch from the Down to the Up, and then back to the Down again!
Plus, everything on the Fasts at Wellingborough (the ICs) can only switch to/from the Slows at Wellingborough North or Kettering South Jns; it's impossible to reach the Corby lines from the Fasts once you pass Kettering South Jn.
That's been normal for diversions since Corby re-opened., i.e. cancel the Corby terminators and replace with calls by through trains. When Kettering - Corby and slightly beyond was still single line (and there was only the one bi-di Slow S of Kettering) there was a lot of (timetabled) hanging around, and about half of trains didn't stop at Corby for pathing. They happened frequently, long before the electrification North of Kettering towards M.H.; at one point, I think before Corby re-opened the last time, they were actively advertised at Northamptonshire stations as an opportunity to have a scenic journey over Harringworth viaduct
There's a slow to Leicester direction crossover North of Kettering, and this takes up all the space before the levels of the two routes become very different. The Leicester route climbs quite steeply towards Desborough summit whereas the Melton route is level . Really shows the gradients on the original route.
The 1962 WTT section A shows trains using the Melton route crossing at 'Kettering Junction', which was South of the station.
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