Real rail is a flat brown color, while model rail is often silver, brass, or a brassy-silver. Some manufacturers of model rail produce different colors of rail, so if they're used adjacent to each other it's obvious that you're looking at a model. Not only will painting the rails look more realistic, but it will also blend the various colors of rails. The rails will then look like they have been together for some time, adding a feeling of continuity and history to your track.
Now that you're ready to paint your rails, make sure they're really ready to be painted. The track should have all the electrical feeders connected, be thoroughly tested, and be unballasted. At this point, any problems can be fixed with a minimum of hassle. Remember: Every rail should be soldered to something. Either a feeder or another rail.
Preparing to Paint
It's time to get the airbrush out and spray some paint on the rails, right? Wrong. Before you start, think about what needs to be masked. If you have live frogs on your turnouts, make sure to mask the area between the point rails and stock rails. Turnouts often rely on the connection between these two rails for electrical pick up, so if they're covered with paint no connection will be possible and they'll fail. If you have any walls or other scenery placed, you'll need to mask them too. Remember, spraying paint covers much more than the intended area.
The next step is to protect the top of the rails. This is the most important part of the rail, as this is where the power is picked up. Applying paint in the very thin layer required to paint the rails can make it extremely difficult to remove. I did not do this on my model railroad and have regretted it ever since. In the future, a thin coat of oil will be applied to the rails before painting to prevent the paint from sticking.
Time to Paint
Once the preparation steps are finished, it's time to set up your air brush and begin painting. If you mix paints, keep a note of what you used so if you run out you can make more. If you're using a single color make sure you've got enough to cover the whole area. Painting a large flat area such as a yard can use up a lot of paint quickly.
As you spray the paint, don't worry about the paint on the ties or roadbed. The paint here will further tie the railroad together and create a unified appearance. You've got all the areas you don't want painted masked, so don't worry.
After you've finished painting, let the paint dry and clean the track. Unmask the masked track areas and clean the rails with mineral spirits. Apply the mineral spirits to a cloth or paper towel, and run your fingers down the rails, making sure to contact the top and inside top corner of the rails. Most the paint will come off easily, but there will still be some difficult areas that will require an abrasive block.
Painted track adds a large degree of realism to any railroad. The paint unifies the track components so it looks like it's been in service for years. With proper preparation and clean up, you can transform your track into a railroad.