I can find that referred to as a "NG" or "Mule." I have seen similar critters near the power plant just outside West Point, Kentucky. I believe they're used inside the plant to shuffle cars around.
These pushers are used at the coal piers to shove cars to the dumper. They are narrow gauge, and the tracks run in between the standard gauge tracks. It's hard to see, but there is an arm on each side below the side cab window that drops down between two coupled cars (or behind a single or cut of cars) and moves the cars forward to the dumper. Older versions used electricity from a third rail between the gauge rails.
very coo(a)l : May this picture helps : http://www.railpictures.net/viewphoto.php?id=330275&nseq=473 Regards Karsten
Those are the ones I am most familiar with, having worked dock jobs at this facility (Toledo). As an additional note to that linked photo, they move more than 2 car cuts. The final move to the dumper is 2 cars, but they can move a whole track (about 20-25 cars) forward for final spotting.