1989 Lionel B6 6-18000 loco gear grind

Is the grease old? Crusty? I'd start there; clean and flush, check action, relube and check again. Old grease gets hard and crusty and just a tiny bit can do it. (or a spec of sand, etc..)

Look at hubs and close quarters for scratching of wheels and axles.

If cleaned and flushed, the noise is going to make "dirt" before other suspect areas.

Motor type? (can?)  

Axle "end play" left to right? There is a sliding relationship with the wheelbase of 3+ axles that allows tight curves... that's why some use blind drivers; a flanged wheel would need too much end play. But worm drives being centered and not sliding helps their efficiency.

or runout play (oblong axle bearings)

Shimming for closer tolerances isn't unheard of at all. For axles; consider a smooth E clip. Easy on/ easy off; no wheel pulling or re-gauging of wheels. They stay put

[ do not have the 1989 remake, but I do have several of the prewar B6 switchers. The only gear trouble I am aware of is with the intermediate gears which drive the outboard wheels off the center wheel. These small gears usually have a polished streak on them where they rub against the drive wheels occasionally.  If these gears are polished across the entire face there is a problem. On the prewar locos the problem is either the stub shaft is worn under size and/or the bore in the gear is oversized.  Or the stub shaft has come loose in the frame casting.  Or both problems exist. The result of this is that the gear can get out of parallel with the wheels and the gear teeth contact the outboard corner of the center drive wheel.  This can get so extreme that the teeth will lock up on the corner of the drive wheel. Hold the engine upside down and move the drive wheels back and forth and see if the small gears cock and/or the stub shaft moves in the frame. If this is your problem there is a rather lengthy post in the Tinplate forum on how to fix these problems.  The title is something like ‘prewar 0-6-0 switcher’.