Sunset ATSF 4-8-4 Derailing

Check the turning ability of that particular truck while the engine is sitting on a flat table. I encountered a similiar problem with my GS-4. I found that the truck was hitting some other detail parts on the engine. A gentle bend of the obstruction cured the problem. Most likely it got bent during the tight packaging and wasnt a big deal, and an easy fix.

I had a similar problem on my 3rd Rail ATSF 4-8-2.  Turned out to be burrs under the head of the large shoulder screw that retains the 4-wheel pony truck, the one that works in the center slot of the truck.  That shoulder surface needs to be free of burrs in order for the truck to lie flat as it swings side-to-side going through curves.  I cleaned and smoothed this surface while spinning the screw slowly in a drill motor.  Solved the problem.

I don't have the 4-8-4...regret that I don't!!...but I would imagine it's retained similarly.  Might check it, anyway...easy to do.

FWIW, always...

KD

I have the ATSF 2900 and the Daylight 4-8-4. I have 072 and 084 curves on my test layout and have not had any problems with the front trucks on either locomotive so far.

I have about 24 hours of break-in running time on both engines.Before running,I did apply a little grease to the shoulder screw/pony truck slot as well as all the drive rods,bearing and so forth.I do recall having a similar derailing problem with another 3rd rail engine,the Santa Fe 4-8-2,I bought this used and it ran rough and the front truck derailed until I serviced the engine thoroughly and gave it a good break in.

Had a problem with the lead truck on one curve with a PRR M1a [4-8-2], but not with the similar M1b.

Called 3rd RAIL and they sent me a new lead truck. Never used it as I had corrected the problem befire it arrived!

Adjusted the wheel gauge [spacing] of the lead truck.

Thanks for the helpful comments, fellows.