I have an HO scale locomotive that constantly on straight aways and my switch studders with its speed. The headlight goes bright and dim and bright and dim again. Why is it doing that? It's just straight and it's nickel silver rail. The part about the switch and why it doesn't run well, is because I think it's nickel silver rail to just plain nickel rail and back to nickel silver again.
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Favorite Answer
Since it picks up its power through the wheels the obvious things to check are that the wheels are clean and the track is clean - the light tells you the voltage inside is changing levels. It may also be that the wheels supporting the loco are lifting the contact wheels from the track because the track isn't evenly level - like pushing a 4 wheel dolly over uneven sidewalk and one wheel lifts off.
GibsonEssGee
You could try bypassing the fishplates to the nickel only rail with a thick copper wire and a couple of alligator clips to test your theory.