Many people have been faced with the question of how to make their rolling stock look faded without doing a complete re-paint with a lighter "faded-looking" shade of paint and then re-lettering.   I encountered this technique by accident while weathering a re-painted SP hopper car.  Mike Rose wrote an article for the January 2001 Railroad Model Craftsman, on this very technique for HO gauge cars, which he also found by accident.  Though his article covered only HO scale cars, it can be easily applied to our large scale equipment as well. 
    The first step in this process is to spray the car with two coats of Testor's Dullcoat.  Let this dry for 24 hours. Next take a pump spray bottle and fill it with isopropyl (rubbing) alcohol.  Give the car a very wet wash of this alcohol including the roof, if there is one.  As the alcohol dries, the color of the car will fade and look chalky.  If you do not care for this effect, you can reverse the whole process by re-spraying the car with Dullcoat.  This will bring the finish back to the original.  Mike had one car that came out blotchy and as such he was not happy with the result.  He resolved this situation with his air brush by over spraying the entire car with a very fine light spray of the same color as the car, but lightened slightly with white paint.  By using this spray he was able to control the paint flow, and it helped to even out the blotchiness that he experienced from his alcohol overspray.
    Other weathering techniques for fading include just using the lighter paint overspray, and also the use of chalks. Mike encourages you to try the Dullcoat/Alcohol method and see if you like it.  If you don't, just re-spray the Dullcoat with no harm done to your model. Mike cautions that if you are doing a locomotive or caboose that has any clear glass in it, to remove it or mask it before you spray the Dullcoat. He also states that Dullcoat is great for making frosted glass with minimal effort.
    My own experience came about when I was putting a black alcohol wash over my hopper car.  The paint with the alcohol wash actually made the car look faded and used, so I left the car alone and did not re-apply any Dullcoat.
Fading Paint Using Alcohol