Older Williams passenger cars

[phillyreading]

Is there a fix for the older Williams passenger(made near 1997) cars that have three axles and the knuckle couplers don't open? They keep derailing every where! What I have done is to cut some plastic near the place where the coupler arms meet the truck frames, this helped with one or two but the whole set of cars will derail even at low or moderat speeds anywhere on the layout.

I put on another set of passenger cars and there is no problems with the track and the other set works great with the same engine.

Lee F.