Another one would be the MML. As I understand it the 4 and 5-car 222s are often pretty full on Nottingham/Sheffield semi-fast workings but as I recall under Midland Mainline these were orginally two-car 170s.
That must've been "interesting"! Though I'm not sure what they replaced, were these new services?
BR: HST every forty five minutes from London to Leicester (every ninety minutes to Sheffield, same to Nottingham), irregular stops at stations between London and Leicester
MM inherited: HST every half hour to Leicester (hourly to Sheffield, same to Nottingham) , irregular stops at stations between London and Leicester
MM introduced an hourly London - Nottingham service and an hourly London - Derby service (with random extensions to Matlock, Barnsley and Burton) that stopped at all stations between London and Leicester so that the HSTs generally ran non-stop from London to Leicester (so faster journeys for long distance passengers)... these new services were ran by two coach 170s and sat at Leicester for around ten minutes so that they were overtaken by the HST (i.e. the London - Nottingham 170 got to Leicester a few minutes before the London - Sheffield HST and departed a few minutes after it, so that you could do journeys like Bedford - Sheffield or a faster journey from London to Nottingham than having boarded the 170 at St Pancras)
MM introduced three car 170s to replace the two car ones (rather than introduce a third coach to the existing ones?)
MM planned four coach 222s to replace the three coach 170s (which meant no more Matlock services as a weak bridge on the branch wouldn't cope with a heavy 222
MM also ordered some nine coach 222s which were intended for an hourly London - Leeds service (seems a long time ago, an era where a TOC would speculatively order new long trains in the hope of being given paths!). I think the SRA were the ones who didn't permit the Leeds service. Shame.
EMT took over, reformed the 222s into four/five/seven coach formations
EMT gained the poisoned chalice of the Corby branch, got the Hull Trains 170s to provide capacity, amended the timetables so that there weren't the same "slow" services any more - everything operated by 125mph stock (and Corby services taking some of the "local" stops meaning the Derby/ Nottingham services could be sped up), London - Derby service extended to Sheffield (running within about five minutes of the existing service due to poor paths at first and not serving Chesterfield due to capacity problems on the line but things slowly improved)
So it's been a gradual set of improvements, same as Hull Trains (who went from 170s to four coach 222 to five coach 180s and soon 5x26m 802s)