Funny you should say that. I always knew 31's as "peddles" ie peddle power aka f*g useless. A 31/4 with 5 on was ok on Peterborough - Birmingham, but if you had 8 or 9 on, you may aswell get out and walk (as it was quicker)!Alternative theory: it was during the great dessert crisis of the late 90s - the remaining fleet were put to work hauling emergency supplies from the Ambrosia factories, but it was soon found that they couldn't pull the skin off a rice pudding, so that marked the end of them.
Getting back OT most 31's ended up in the "engineers/civils" sector on privatisation (dutch livery and the orange and yellow squares come to mind). Nearly always operated in pairs (because they needed to be to lugg rail and ballast etc around) and nearly all ended up being scrapped!
To be fair, 31/4's with 5 on was ok to trundle along at the same speed as a dmu (ely - norwich used to be class 101's at 70mph) and would happily keep to time. No good sticking one on a Norwich - Liverpool St though. And certainly not the "European" !indeed
I was going to suggest there are a couple of them on New Street - Norwich trains that are still out in the fens somewhere.
I can remember the European calling at platform 3 at Ely with about 5 coaches hanging off the platform ! And no one ever opened a door and fell onto the ballast !! Hooray for 'elf n safety. Much needed in some parts of industry but by jolly it also means that generations younger than me have poorer common sense because of it.
With the aforementioned services above what 31/4's were primarily used on, the traffic didn't demise, as the services went over to "Sprinters".Yes after rebuild with EE power units they were pretty reliable and were perfect for short engineers trains/ speedlink and some passenger trains. The ETH fitment in the 1970s made them very useful as well. They were under powered compared to a class 37 but no worse than any other type 2. They disappeared in the late 90s as they were over due overhauls coupled with the demise of some of the lightweight traffic they were used on.
In the case of Cambridge - Birmingham N S. the service went 31/4, Class 150, 156, 158 and finally 170.