For the Ferroequinologists (and Boat People, Too)
Original LoC caption:
Exhibits the surface of a pair of barges, showing the tracks for loading and unloading cars, also the movable bridges by which the tracks on the floats are connected with those on the wharf
These are ramps used for loading and off-loading railroad cars, ferried across a waterway on barges. The date and location aren’t given, but it’s at a sizable town, perhaps one where the rail bridges had been destroyed and this is a temporary attempt to provide transportation across the stream.
The locomotive shown here is a bit long-in-the-tooth, dating at least to the early 1850s, and maybe a little older. It’s been relegated to switching duties, as evidenced by the removal of the pilot (“cowcatcher”) at the front, replaced by a regular, link-and-pin coupler. I think the tender carries the letters U.S.M.R.R. (U.S. Military Rail Road), but it’s hard to make out. High-res image here.
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Dead Confederates, A Civil War Era Blog 
Based on the size of the ships’ prow in the background the port must be able to handle ocean going vessels. I also see white blotches among the buildings, melting snow perhaps? Just a guess.
Agree on the size of the vessels; dunno about the snow. I’ve found two other images in this series, that indicate this was probably at Alexandria, used to ferry rail cars down to the army at Aquia Landing.