Newly returned to hobby and now I have a partially complete layout have tested my ten Bachmann 66s and am experiencing this problem on 66152, 66731 and the 66623 (Ltd Edition). 731 was brand new; other two mint s/h.
152/623 derail in both directions on most points and seems that it is probably that the leading wheel derails on point frogs on the 'curved route'. 731 seems to only be affected on one set of point when the leading wheel derails when about 5mm after the tip of the switch position when loco running in direction from frog >>>switch
tips. All my other loco's with 6 axles are OK eg Class 37s.
Whilst I understand the basic concept of the tips to correct I would find it helpful if someone could provide a sketch please - age related eyesight not helping!
Has Bachmann done anything about this problem - clearly been going on for a long time? Lastly I have heard from a N gauge colleague who has experienced the same with Farish 66s.