I have a problem with the Eureka 620/720 couplers when the power car is pushing the unpowered on curves, 1 metre diameter, it derails.
I think it is because the ends are touching, I have extended the couplers by adding a short brass strip but this has not worked.
Does anyone have a suggestion on how to modify?
Have thought Kadees but mounting is not the easy or a solid strip with pivot in the middle.
I don't own a Eureka 620/720, however I have a couple of observations/suggestions for you.
1). Have you tried using a different loco to push the unpowered car over the same section of track?...or even push the car by hand? If this works okay, then that eliminates one possible problem. However, if the car still derails, then we can safely say that the problem probably lies within the unpowered unit.
2). Do the bogies of the U/Powered car swing freely enough such that they are not being forced to de-rail by virtue of them snagging on the "skirts" of the super-structure?
3). Do the bogies, when being turned on their pivot/s, remain horizontal? By that, I mean is there any distortion in the angle of the bogie in relation to the remainder of the car, as the bogie pivots on it's centre point?
4). Have you tried temporarily adding some extra weight (on the roof would do) as an experiment? It may just be a case of needing to add a small amount of weight over one/both bogies to avoid the U/Powered car from jumping the rails.
5). Does the de-railing only happen on one part of the layout? It could be that there is a slight "hole" in the road and the bogie/s do not have enough lateral play to enable it/them to conform to the irregularity. You may be able to back-off the pivot screws in the centre of the bogie/s, to enable them to ride over such irregularities with de-railing.
See how you go with some/all of those ideas.