The easiest way I've found (which also avoids using anything abrasive on the wheels, which itself can cause future problems) is as follows:
Take a double or treble thickness of paper towel perhaps three inches by one inch, and soak it in track or wheel cleaner. Then with one hand hold it across the rails, apply power to the track, and with the other hand gently hold the locomotive so that one wheelset is rotating over the paper while the other(s) are picking up power. Repeat to taste, moving the paper around so that the crud gets evenly distributed on it rather than just passed from wheel to wheel. Works every time, and costs almost nothing!
John M Hughes West and Wales Web at
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