Crayuft is part of the cover-up! With his well-known techno-skills, he has cleverly altered the photo in his facetious post to make it appear as just another innocent, pre-1940 mixed train on its way to Santa Fe.
However, a quick search of the Denver Public Library archives turned up this blow-up of the REAL & unretouched photo of that same train.
One can clearly see "Fat Man" tied down to the roof of the box car on its way to Los Alamos, where Oppie and his boys will stuff their new atom splitter into it.
So now what do you have to say for yourself, Crayuft?