Where are the slow line stoppers terminating? If Tring, then are there really going to be 8-12tph of services from Hemel Hempstead to Tring on the slow lines and freight paths. Moves on the flat Bourne End Junction need to be taken into account as well.
The clever thing with the current timetable is that the crossing move at Ledburn is north of where half the stopping service has already terminated.
If the fast line regional trains are every 10 minutes and call at Watford, how many stops could a semi fast make on the fast lines before they'd get in the way? 1?
And if they were flighted a bit more would something like this work on the fasts:
xx00 Birmingham ("Fast": WFJ, MKC, Rugby...)
xx05 Trent Valley ("Fast": WFJ, MKC, Rugby...)
xx08 Northampton ("Semi Fast": WFJ, Hemel, Berkhamstead, slows at Ledburn)
xx12 MKC ("Slowish": WFJ onto slows, doesn't call everywhere from there onwards)
xx20 Northampton ("Fast": WFJ & MKC, slows at Roade, continuing Birmingham to alleviate platform use at Northampton)
xx23 Northampton ("Semi Fast": WFJ, Hemel, Berkhamstead, slows at Ledburn)
xx30 Birmingham (cycle restarts)
That's a mere 12tph on the fast lines; I'd imagine the slow lines would be all shacks to Tring, though there are 6tph that move over from the fasts at some point, and freight to path in that lot too. The exact destinations of the "fast" trains don't really matter that much; might be better for the xx05 to be the Northampton and Birmingham one to spread them better north of Rugby and the xx20 to go up the Trent Valley.
Crayon timetables are hard, kudos to those who do it for real and have to think about the up lines and junction conflicts as well!