MTH Engines on Fastrack

It could be the gears on the trucks. There's usually some lateral playing the powered axles that can push gears to one side of the contact area and create gear noise.

Its most noticeable in one direction because the gears are on one side and the noise is made when the gears are offset to one side or the other.

Wheels hitting the ties would be a slower clicking than gear noise. Pickup rollers can make a clicking noise as well and would be slower still.

Posting a video would be helpful to narrow down the noise.

I second looking at the pick up rollers.  They can definitely cause the clicking and simply need to be oiled per the instructions included with the locomotive. 

Thanks guys will check the gears for slop, and make sure the rollers are properly lubed.
ok rollers are fine; there is some side to side play on the axles of the RS3 and that might be the problem, but, there is an equal amount of play on the axles of the FM, and it makes no noise at all.

I’m leaning toward a gear mesh problem.  Try inserting a trimmed bread wrapper closure on the axle to reduce play and improve gear contact.  There just happens to be a recent thread on the bread wrap closures.

Do You Use Plastic Bread Wrappers To Improve Gear Alignment? | O Gauge Railroading On Line Forum (ogaugerr.com)

Had a K-Line interurban that the gears built into the wheels would hit something on a K-Line switch, almost as if they didn't QC the design. Converted it to non-powered and used my mini-lathe to reduce the OD of the gears.

Thx guys. Have tried to wedge a trimmed bread closure, and it will fit, but I can’t get enough force down to get it over the axle. Will try the nylon washers when the hardware store opens tomorrow.