If this long discussion has proven anything , it is that the WCML (particularly south of Rugby) , but also Crewe - Weaver Junction , and probably Preston to Carlisle is an absolute challenge to timetable. Apologies maybe for some gritty replies from those who have done real work on this critical section , - but it really is a deep challenge, and options for squeezing more into the route is very far from easy. It just gets busier and busier.
Two comments....
(a) A couple of 4 car workings into Euston around 0930-ish , is because by then you have run out of units - most 8 / 12 car sets are at the London end - with no time to run out and do another journey. This was often compensated by getting some Watford Junction peak "bouncebacks" - giving some relief to the still busy inwards flows. Making the terminating bay a 12 car (as has happened) helps. Part of the peak problem is getting rid of high peak rolling stock. (in one 1998 timetable , we were so desperate , we considered a 6 car 313 to help out - not a good idea frankly)
(b) I took an affable "La Vie du Rail" journalist for a cab ride on a 350 from Euston - visit to Rugby SCC - and a ride back on a 390. He was amazed when you had all 4 tracks occupied at several moments - a 390 hammering down the up and down fast lines , us on the down slow -and a string of Freightliners on the up slow , mixed with "suburban" services. His comments were "France is not like this" - but he lived in Lille and was used to the HS links to and from Paris , where he commuted every day" - opened his eyes for sure , and he wrote a great article.
A very nice touch was that RY SCC had a fluent French speaker (of that origin) , and they made sure he was on duty for that visit , so he could discuss in detail what the issues were.