Don't use a finger, but probe the slot for burrs. Edges should be softly rounded to top/bottom; not "sharp". Same for the truck hook/tab, soft edges slide, sharp ones bind.
Maybe bind from/in the blue paint thickness (a slight, even, "run" in the paint where sliding occurs).
Have you.looked at the wheel gauge close? Will flange backsides clear the guide rails well? Is a wheel hopping into the gap though it's rubbing the guide? (wide)
Are the trailing truck wheels of fast angle design? I wonder if a set of flatter treads might be a better fit for the Atlas railhead?
Is the rail gauge ok? Aligned inline across gaps?
Any possibility the down force spring saw heat? Derail, etc..? (heat kills some springs pretty easy)
