It looks great; nicely done.
I see weathering like this on such a relatively new car to be more dust and grime. There could be any amount of crud on the car. Like Dave Vollmer said, blackish/grayish roofs work, particularly if you've got tunnels on your system, from either steam engines or first gen diesels working hard. If there's a serious rainstorm, the crud could be mostly gone the next time you'd see it.
If it was overall fading, I'd expect to also see bits of damage to the white lettering, perhaps a touch of run if stencil paste was used. Lettering often wears unevenly.