There are tons of dark/semi dark textures that will easily do that, by setting the scale to minimum, and the radius to @ 9:00 ... and blurring textures holding down on the KB [ ] keys while painting.
Sadly, although the textures are arrainged alphabeticly, one great coal textture might be named Asborenhuagh_1 and he next great cinder texture named WorhausGesisightnicht_Dark
You sorta' got to make a "painters pallet" of all your favorite textures, off on an unrelated spare baseboard, and lay all of the textures one by one, until you find the right one for your needs. You can also write them all down, in pencil, in your favorite Trainz tool: The Marble Notebook, or make a complete seperate route called: "Test Textures" for easy referance.