In 1948 Lionel was in the process of cutting the tooling for the all new NW2 but they needed a stand in for the model for the catalog shots. They purchased a hand full of General Models NW2 switchers which were actually closer to scale than what Lionel produced. Die cast shell, fixed pilots, more scale proportions, etc. Anyway, Lionel painted a few of these up in a basic white scheme with Lionel Lines on the hood and the circle L on the cab sides. It earned the nick name "The Phantom" because it was only seen in a hand full of promo photos on the 1949 showroom layout. It was covered pretty extensively in CTT back in August of 1992.
The one in Mr. Young's sale is painted black.
The title of the caption calls it a prototype. But the details do describe it as a GMC engine used for photographs. I wasn't able to read the text earlier. Don't know why I was able to do so today.
