Sunrise, try this:
take a random sampling of several standard gauge cars, preferably from different manufacturers, vintage and reproduction. place them on a straight section of track. grab each car and try to slide it (gently) sideways, back and forth. not lengthwise so it rolls on the track, but sideways, cross track. as you do so, get your eyes down on wheel level and watch the trucks and wheels.
you will find that some cars will fit tightly with their wheel flanges between the rails and slide sideways hardly at all. others will slide back and forth 1/4 inch or more - their wheels may come close to coming off the inside of the rail.
this is normal. these are tinplate toys, and the manufacturing tolerances are just not that close. a sixteenth of an inch one way or another is well within tolerances. which is why the MTH rep was trying to be polite, but there really wasn't much he could say.
the trucks are made out of thin sheet metal that bends in your fingers. if the axles appear to be "too long" and are binding, just spread the trucks a bit with your fingers. you will be surprised how easy this is, you may easily overdo it and have to bend them back. if the axles are "too short" and fall out of the trucks, squeeze the trucks so they bend inwards a little, and pop the axles back in.
as with the "rookie question" tinplate thread, the relevant answer here is "slop". tinplate toys are just not made that exact. wheels, trucks, couplers, all flop around. there are no bearings, just oversize slots and holes. if things aren't running right, the fun is in diagnosing just what the issue is, and tweaking things a little, sometimes just with your fingers, sometimes with a pair of pliers.
this is normal. tinplaters have been doing it since around 1910.