I have the misfortune to travel on Northern's trains on a variety of routes on a regular basis. Whilst it is true that they do have some "nice" trains with clean and well maintained interiors, sadly it is also true that the vast majority of Northern's stock is quite disgusting.
Forgetting pacers for one minute, all of the 158s are very tatty, filthy inside and out and have disgustingly tatty and smelly toilets - even when clean. The various cheap refurb jobs have produced a variety of different seat cushions with ill fitting and badly installed seat covers, many of which are coming off. The carpets (what remains of them) are threadbare and in places the floor underneath can be seen. Some of the carpets are central, some wales and west, and some still regional railways. Compared to the refurb jobs on other TOCs units, they are a disgrace. Oh yes and most of the air con is defective.
The 156s which were once nice trains are also disgusting, although they have started a bit of a refurb job on them recently and the refurbished ones aren't too bad - at the moment.
As Northern's cleaning standards are virtually non existent and just seem to involve a litter pick and that is it (even overnight) then it won't be long till they are horrible too. I have not been on one 156 with working heating for a year or so.
The 333's whilst relatively new are getting in need of a freshen up, but overall aren't too bad.
The 150s vary considerably. The ex LM 150's are significantly and noticeably cleaner and in better condition than the Northern 150s. Despite the fact the ex LM units retain their original brown interior and old floor, and in many cases their central seats, they are overall far nicer to travel on - you stand less chance of soiling your clothes on them.
The 144s are quite clean and well looked after. In general the 142s are not, although the ones which have returned from FGW and still retain their FGW seats are noticeably cleaner and better maintained overall.
There are a number of reasons for this. Firstly the quality of overnight cleaning at many locations is poor. This means that trains enter service already a bit dirty. Over time this builds up and they just look tatty no matter what time of the day it is. Secondly The refurb jobs that most units have had is a cheap and cheerful one mainly consisting of poorly fitted seat covers which fall off and look tatty. Nothing else has really been touched. Many units have not been touched throughout the life of the franchise and are very tired indeed. Thirdly, maintenance standards of cosmetic items seem poor and many things seem bodged.
While it is true that many of the trains are old, that is no excuse. Old trains can be just as clean and presentable as newer trains if they get the right treatment. I find it is often better to avoid sitting too close to the walls or putting feat underneath seats as you can quite easily end up soiling your clothes.