I just noticed in the corner of this picture of the turntable and engine house at City Point that there is an ambrotype studio.

Turntable and Engine House

A high res version can be downloaded front the National Archives or from Wikipedia.

A few observations. The wagon isn’t a typical military wagon, so I suspect it might be a support wagon for the photographer. The Ambrotype sign written on the side of the railroad machine shop on the left side of the image also is a bit puzzling. I doubt that the army would allow a private photographer to advertise on the side of their building. The most likely explanation that I can think of, is that this is actually A.J. Russel’s studio, as he was an army captain tasked with documenting by photograph, the US Military Railroad during the Civil War.

Here is a crop of just the Studio. Notice the north facing open roof. Before the advent of high intensity lighting and flashes, photography studios typically would have large north facing windows to indirectly light the scene. Looks like that in this case, they just opened up the roof.

Ambrotype Studio at City Point

The remaining question, which I can’t answer, is whether Russell was also doing some freelance work while at City Point.