This photo, via the Library of Congress, shows part of Alexandria’s wharf which was used by the U.S. Military Railroad Department. For more about Alexandria Virginia’s waterfront, see this post from the Office of Historic Alexandria on Alexandria’s First Wharf (PDF file). See this post as well on a timeline of Alexandria’s waterfront (PDF file).
The U.S. Military Railroad would have taken the goods from the waterfront up the Wilkes Street tunnel and toward the west end of Alexandria near where Carlyle is located now. For more information about the U.S. Military Railroad, visit this link for a terrific blog on this.