On my HO layouts, I spray all of the track (rails and ties) with flat black, or Rustoleum camo paint as a first layer. Then, I paint the rails brown and I paint individual ties various shades of brown, gray, and black to represent old, weathered ties. I would suggest spray painting all of your track flat black first, which makes the middle rail the same color as the outside ones. This simple step alone is better than just all-silver track with a black center rail (neither of which looks realistic). It also kills the shiny wood or shiny tinplate metal tie look
If you want to paint the outer rails, or all three rails, rust brown... that can be the next step.
