I have found that MTH diesels with smoke produce a fair amount of smoke in neutral with maximum power applied. However, when running at 10 to 12 volts, the smoke is either nil or barely noticeable.
I would love to know a smoking diesel that produces obviously noticeable smoke when running at 10 to 12 volts. Sometimes my Lionel Legacy D&H RS-11 dies that and sometimes it does not.
My guess is that it is important to put in just the right amount of smoke fluid in order for the smoke to be good at normal running speed.