It's so funny coming from a theater background, where we almost always dirty it up...or at least add some texture.  Heck, even back in my model railroad days, it was all about weathering...you didn't consider your new rolling stock done until you'd stained, added mud splashes, and so forth.

The second product I sold here had layers and layers of sun-bleaching and staining in the texture maps.  Oddly, no-one made the slightest comment (pro or con).

But, yeah, that's something that was drilled into us in theater and is in every 3d texture book I've ever read....no flat colors, no flat colors, no flat colors.  You always break it up a little.

(And, yes, I realize that's not the same thing as being dirty and beat-up.  But good weathering will always be about naturalism.  Mud doesn't splatter on the tops of your boxcars, after all.  It gets thrown up from the wheels.  Regardless of whether you are going for "nearly new" or "practically abandoned" you have to follow the history of what happened and how that dirt and those dings got there.)