It's probably the easiest and cheapest improvement that you can make to the appearance of your track. I used PollyScale, and applied it with a brush. Make sure that your trackwork is sound and that it's not the cause of derailments or other operating problems. Switches (turnouts) take a little more time but plain track is a breeze. Use a good quality brush at least 1/4" wide, with firm, but not stiff, bristles. As mentioned, just slather it on; don't worry about getting it on the tops of the rails or on the ties, but do make sure to work it around the moulded spike heads. After I've done a switch, or 12' or 15' of plain track, I use a dry rag over my finger tip to wipe the railtops. The paint, while dry, is not hardened, and wipes off in a couple of passes. Any excess on the ties won't be noticeable after you ballast. (My ties are brown, the same colour as I used on the rails, more-or-less). Even if you're using a rusty colour on black ties, the spike heads, tieplates, and the adjacent area of the ties all get discoloured on the prototype.
This area was done with the brown, then I later went back and did the spike heads and tieplates in a rusty colour: wasn't really worth the effort of the extra step.
Some of the rails here got the brown, while others were painted with an oily black. Either one is a big improvement over shiny.
Wayne