Pity the wheel are causing problems.
my Cravens are 10.5mm so you would need this size if replacing the wheels.
Try the wheels on another coach to see if it wobbles.
Nice wagons, good work. You could consider having some loaded up.
Maybe with a false floor near the top, so you only need a small load to cover the false floor.
Great job.
trying to make things fit is hard.
The drive shaft spins so fast that if it is not totally straight it will make noise, try making another one
to see if it is any better. The noise could be coming from somewhere else.