In The American Heritage History of American Railroads by Jensen, this 1862 image shows a bridge under construction. Major General George McClellan of the Union Army brought locomotives and cars by ship from Baltimore and ran trains as close to four miles to the Confederate capital. The workmen are seated, and to the left is a photographer’s field darkroom. At that time, photographs had to be developed immediately and while wet.
To their left , a locomotive was arriving on a ship in White House Landing on the Pamunkey River.
Here is another image of the field darkroom, invented by Matthew Brady.
The wagon would carry the chemicals, glass plates, and finished negatives. Can you imagine what would have happened if the horses got startled or took off at a gallop?