The tricky thing is that last week, they WOULDN'T do that.
I ended up using a jewelers file and sanding and cleaning up all the moving parts (the swing arm, the underframe "bracket" and the insert with stirrups.
Before, I found that small burrs and molding remnants were causing the trucks to "stick" in various positions. Most amusingly, they would stick in the "turned" position, and derail when the car went back onto the straight track.