Almost always too much?
I've been working in Preston for a few months now (Live in Edinburgh) and very rarely see a TPE that isn't bordering on chocca whether it's 3, 4, 6 or 8 cars.
From Wigan onwards towards the North though the difference in pricing between VT and TPE makes them much more attractive compared to VT. as the TPE advances as usually always cheaper than VT which adds more people onto an already cosy service.
You can't blame people for picking the cheaper option though, I do it myself and for the TOC it's all about revenue.
Interesting subject been exposed to recently. What you say seems part of the problem, and this is mixed with one other, which is commuter use between Preston and or Bolton and Manchester.
In a nutshell, Prestonians(?) pack the trains out back home, where they empty out and then this gets filled up again with people joining the train towards Scotland.
It's a fascinating picture given all it merges and the questions it raises, such as
Does the crowding up to Preston put people off travelling to Scotland from Manchester on these services? How best to cater for Manchester to Preston home time traffic. Arguably better and more appropriately served by 319 type services, same as towards Liverpool?
Given the loading and unloading at Preston, would it be better to have four carriages from Manchester to Preston joined by another four from Liverpool, where it is the case that the lack of direct services will suppress demand, and is where a lot, perhaps the majority, of passengers joining at Preston have originated.
Complicating all this are those ticket sales, advance versus walk up, and the purchase of advance fares both from Wigan for VT and Preston for Transpennine by passengers from Liverpool whose original starting point was Lime Street station (uncounted/miscounted present day demand). The Transport for Scotland response to the consultation was interesting.