What Bob said. Some had flywheels that were out of balance, in some cases because they used single set screw. All of these used a fairly high-rpm motor combined with a low gear ratio (the best part!) However any vibration is amplified by the brass shell. There's a reason they don't make band instruments out of die-cast metal!
If you remove the flywheel the loco will still run (assuming the motor can be connected directly to the gearbox.) But it won't coast. The loco will stop on a proverbial dime, and if you mishandle the throttle it might result in a derailment, or even damage to the loco itself! Instead, you could try installing something called Dynaxorb inside the shell. I did this to one of mine and it quieted it right down.
Factory rubber tires will probably be rotten at this point, so you'll have to figure out which MTH locos had similar-sized drivers, and get new tires from MTH. Detail on all of these was sparse, and realism is further reduced by shiny handrails, driver tires, etc. Paint and weathering greatly improves the appearance, as would Precision Scale detail parts. None of them had factory sound or command control. Adding it will require insulating the tender shell from its frame, because radio signals won't penetrate the brass bodies.
Finally, the very earliest Williams locos (pre-1987) weren't especially well-made. They turned the 3-rail brass business over to Weaver in 1990 just as they were getting good at it. Definitely a bargain at current prices, but it will need some mechanical, cosmetic, and electronic upgrades to equal the best of today.
