4 or 5 car Meridian on MML under Stagecoach has ever increasingly become the norm, but many still think they're an absolutely wonderful TOC. Hope new operator on MML will see a return to full length InterCity stock on all services, whether express or semi fast, as well as a much improved frequency and faster service on a Sunday instead of the current dire offering.
Worth putting a bit of context on things:
Midland Main Line services were every ninety minutes (each) from London to Nottingham/ Sheffield until around 1990 (with HSTs slowly replacing Class 45s).
They were then increased to every sixty minutes (and remained at that frequency when National Express won the first franchise - the slightly confusingly named Midland Mainline - i.e. two words)
National Express bought thirteen two coach 170s to provide stopping services every sixty minutes (each) from London to Nottingham/ Derby (as well as occasional extensions to Burton/ Matlock/ Barnsley). These services stopped regularly at stations south of Leicester, rather than the haphazard stopping pattern that the HSTs had - and also allowed the HSTs to be sped up. However these services were designed to wait ten minutes (?) at Leicester, connecting with the HST services, so very slow all the way from London to Nottingham/ Derby
National Express extended most of the two coach 170s to become three coach 170s
National Express replaced the coach 170s with sixteen four coach 222s (and ordered seven nine coach 222s for a proposed Leeds service that was never granted paths)
Instead the twenty three 222s were reformed into five and seven coach lengths (with, I think one or two four coach trains remaining due to the imbalance in carriage numbers) and the "slow" Derby service extended to Sheffield (though initially with terrible paths, no time to stop at Chesterfield much of the day, running within five minutes of the existing London service)
Additional 222s were brought in from Hull Trains (partly to permit the Corby service to run), and later some six coach HSTs from Grand Central
So, unlike a number of other TOCs, "full length" trains haven't been replaced by "half sized" trains - if anything the "half sized" trains have replaced "quarter sized" trains!
If things don't look perfect today then they look about as good as they've ever done on the MML (which has always been a secondary route in my lifetime). Yes, I'd like longer trains, and more Standard Class seats on the badly ratio'd 222s but we've come a long way, baby!
To keep things simple, I'm ignoring the period when Nottingham went down to just an hourly service (with the "slow" Nottingham path being used for the Project Rio service to Manchester)
(and, as usual,
@edwin_m has explained things much better - and much faster - than I managed!)