Alexandria National Cemetery has an online database at this address...

http://www.interment.net/data/us/la/rapides/alexnat/index.htm

I used the term "1864" in the "find on page" search feature in internet explorer and just in the listing for surnames starting with "A" found over a dozen graves. If one took their time I imagine could learn a lot from this site. Further questions on the cemetery itself may perhaps be answered by a phone call to the office of the cemetery. Directory Assisitance should be able to provide that answer.