Are trains "point sources"?

So argues the Sierra Club, et al., in an intent-to-sue letter sent to the BNSF Railway and others.  The environmental groups contend that the railroad’s cars discharge coal dust and other pollutants into various waterways.  I’m not familiar with any decision expressly holding that train cars are point sources (a necessary element to liability under the Clean Water Act), but the statute defines point source very broadly, and I would not be suprised by a court agreeing with the environmentalists.