I use Rustoleum camouflage ultra flat paint. Put a light coat of oil on the top of the rails and spray away. Just wipe the top off when finished. It is an excellent ground color brownish and it gives a realistic heavy dirt/rust color to the track and makes the ties look like real wood. You can do this before laying the rails but I do it after and the spill over is perfect for when I add ballast and ground cover. You can use a piece of cardboard to keep it from hitting other objects.
Dennis Brennan, forum member, wrote a good book about "Realistic Toy Train Modeling a Hi-Rail guide" which has an excellent section on trackwork.
Here is a picture on my O27 weathered track. Track was sprayed in place.