Welcome to The Gauge :thumb:
There's some things to do;
1. If the trucks are mounted with screws, tighten one set up 'til it can spin left and right freely---but not swivel. Then tighten the other one enough to do both, spin and swivel. This gives you a three point suspension that will help.
2. Change out the wheelsets to Kadees, Intermountain, Atlas or Proto 2000 metal wheelsets. Much closer tolerances are used to make them roll with no wobble.
3. Add weight, 1/2 oz. plus a 1/2 oz. for every inch of length. A standard boxcar should weigh about 3-4 oz.
Hope that helps