There are a dozen different methods that all work well. I use 91% isopropyl alchohol on a paper towel wrapped around a wood block.
For stubborn buildup I use the abrasive eraser in Lionels track cleaning kit, or a "bright boy" abrasive block. then I go back over with the alchohol.
I only clean my track a couple of times a year, or when trains start acting up. Just beacuse the track doesn't look dirty, doesn't mean it isn't. There can still be a layer of dust/grime that accumulates over time. When this happens. The top of the rails are dull with a haze, as opposed to shiny.
