Well I received my class 91 durham cathedral just over a week ago, looks great but sadly derails on my 2nd and 3rd radius curves with inclines and at some points . I checked a number of my other locos and they are all fine. Yes, some show a slight wobble but nothing horrific and no other derailments, on close examination i am convinced its the fore/aft inflexible bogie issue mentioned earlier , but I suspect exacerbated hugely by the weight of this unit.
i also have a couple of 2020 apt power cars which are not very forgiving ( indeed i tweaked my layout to accommodate them) both these locos have bogies which pivot fore aft when not pressed onto the main body, but move very little if anything when pressed ( held toward the body) my other similarly bogied locos ( modern Bachmann , older Hornby or very old triang) all retain some significant forward / aft movement when pressed) of course the vast majority of them have much of the weight ( the motor) directly on a bogie and not pressing the body down onto the bogie.
My APT power car handles the curves etc ok, but the class91 will not, the class 91 is a little shorter ( but surely that should if anything help) The only other difference is weight, the class 91 at c600g is 50% heavier than the apt at c400g. Both have most of the weight in the body so its borne by the bogie/ body joint.I suspect this “locks” the bogie more firmly and making the loco very sensitive to imperfect track laying, in particular tighter (2nd radius) curves on incline changes and points.
so, sadly i will have to send it back tomorrow ( internet purchase direct from Hornby arrived 11 days ago) interestingly one of the people i spoke to at Hornby mentioned its known to have trouble with 2nd radius curves, pity it does not say that in the advert, on the box, or the instructions.
So i think i put this one down to what some consultants at work used to call “suboptimal” design. Although I’ll never know if my tracklaying was perfect, would it have coped!