To add to this, although the planning rules permit it, it has been proven that there is no slack in the margins in the planning rules if crossing at Kettering South (if timetabled correctly) E.g obviously if the Corby is late it will delay the train behind, but also if the Sheffield train is early it will be delayed because the timings take no account of the slow down required because you are running early. This delay is then immediately knocked onto the Nottingham behind because of the three aspect signalling.
It has also been proven that the adjustment allowance for the crossing move to the Down Slow at Kettering South is insufficient, and combine this with the existing adjustment gets put in the wrong place, you basically end up with a non compliant timetable, even if you look at the timings and it appears it is. All this then used to get very messy when the Up Nottingham was late, often the Corby would be held at Kettering South awaiting it to pass, with the Sheffield and Nottingham queuing behind.
Thankfully this is largely resolved now, the Weekday off peak was resolved in the December 2024 timetable (as part of this the dwell at Kettering in one of the Nottingham to London trains was increased to 2 minutes). Most of the Saturday has just been resolved in the timetable change this weekend. Hopefully in the coming years we will see the planning rules updated which will make crossing at Kettering South less of an option because it will have knock on effects further north due to the increased diverging headway value.