I have an Athearn Genesis SD75M that derails one axle when it hits a tight curve in a tunnel (arrgghh!). It only does this in one direction.
All my other Athearn trains (older U28 and U33, and two “newer” C-44-9W) do not have this problem. It seems that they have more lateral play (ie: are “looser”). How do I loosen the trucks?
Otherwise, it runs great – VERY smooth, lighting is fantastic and the acceleration is excellent.
The Blue Box and RTR Athearn have more rotational and lateral motion due to having less details.
If you want to run those highly detailed prototypical locos, you’ll have to get bigger radius curves, and make sure that the curves are free of kinks (both vertical and horizontal).
Figure out what is restricting the truck motion, and remove it. It may be details, rotational stops built in to the frame or trucks, or something else.
24" is a reasonable min radius for this locomotive.
I had similar problems with Athearn Genesis SD70 models continually derailing on a 26 inch radius curve, but only when going forward. If they were running in reverse, they made it through the curve with no problem. Fortunately, this particular curve was not in a tunnel like yours is. After running the locomotives several times through the curve, in both directions, and watching the trucks very carefully, I discovered that there was a slight dip in the outer rail about half way around the curve, and the locomotive’s front trucks were not flexible enough to follow the contour of the dip. The solution was to carefully shim the track all the way around the curve to insure that it was absolutely level and had no dips or rises. After that, I have had no more derailments. The problem seems to be that the front trucks on the Genesis locomotives are too stiff and don’t have enough free play to conform to irregularities in the track. I have read comments by other modelers who have resorted to cutting or filing away parts of the trucks or bodies to give the trucks more flexibility, but I didn’t want to take that approach and ruin a brand new locomotive. Another member of the club where I was running the Athearn locomotives had a similar problem with a Bachmann Spectrum Amtrak Acela, but on a different curve. His problem, too, was inadequate flexiing of the trucks to conform to track irregularities. We now use these two models to test new trackwork – if they don’t derail, nothing else will.