Most HO metal wheels have one wheel insulated. You have
to look closely at some to see the insulator.
So, unless that insulation is compromised, they shouldn't
ordinarily short or throw large sparks.
However, some cars may have METAL trucks. If metal
wheels are used in these, all insulated wheels must be
on the same side of BOTH trucks else there would be
Also, if there are wipers on the wheels for one side
of the circuit and the metal trucks and car are the other
side as in some tenders, there could be a potential