On the Nantucket Railroad display I decided to use the Bachmann On30 Forney as the display motive power. The Forney is a close match to a 3-foot gauge Mason Bogey engine the railroad leased from the Boston, Revere Beach & Lynn RR.
The Bachmann On30 Forney ran great for two years. When "crash-bar" type handles were added to the front of the layout, to give children a hand-hold, the track went out of level and the rear truck under the Bachmann On30 Forney tender would hop on and off the track. I tried all the tricks to get the engine to track better.
Finally, I decided to try solder wrapped around the rear truck's plastic axels. I used the thinnest solder I had and wrapped the axels keeping the solder below the wheel diameter so the solder would not foul on track or guardrails.
It worked. The rear truck was heavy enough to track properly and not hop of the rail. Another two years of everyday operation.