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3 Responses to Back Wheels Derailing
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I don’t know why the fronts wheels don’t do it, but it seems obvious to me that the flanges on the back wheels are changing the position of the point blades. This could be due to weak tension on the points. If tightening the tension doesn’t cure it, then try cutting a small depression in the inside of the stock rail where the point rests against it. That would allow the point to nestle inside the stock rail and keep the wheel flange from pushing it around. Good luck!
If only one wheelset out of all of those on your train is proving troublesome, then I would suggest you need to look carefully at that wheelset. Confirm it runs true, without wobbling and check the back-to back measurement is correct (the NMRA, for instance, publishes standards for this kind of thing). It is possible that the point switch blade(s) is/are slightly bent or, as Bob has already said, the over-centre spring that is supposed to keep the switch blade hard up against the stock rail isn’t up to the job. Does the problem occur no matter which way the point is set? If so, you would need to check both switch blades as well as the spring. One diagnostic trick you could try is to hold the switch blade hard up against the stock rail as the train is passing, by pushing or pulling (as appropriate) on the tie bar. Peco point tie bars have a hole in each end (in N Scale, at least) to facilitate the use of “wire-in-tube” point control, so you could use a suitably bent piece of stiff wire or a very fine jeweller’s screwdriver, for instance, to achieve this. Presumably there is no problem as the loco negotiates the point in the opposite direction.
Rob, by vintage I’m assuming you are referring to brass turnouts. I use to have them on my original layout. They have one piece points that pivot on a brass eyelet rivet (grommet) and a rivet that secures the points to the throw rod. These can loosen over time and cause the points to float up and down. This causes the points to rise and gap when the weight of the lead wheels on long trucks reach the heel/pivot point and let the trailing wheels follow the wrong stock rail or just fall down between the rails. The brass strip between the points can also bend at the rivet which causes the points to be higher than the stock rails and slightly reduces the distance between the points. This can cause a small gap at which a sharp thin wheel flange can derail. If these are the type of turnouts you have, you can try to tighten the rivets by placing piece of thin hard metal between them and the roadbed and tapping the top of the rivets with small hammer and a metal rod or rivet setter just slightly larger than the rivet being careful not to bend metal connecting the points. The other option is to replace the turnout with more modern one. I hope this helps.