I've done some extensive research on this supposed railroad gun at Cold Harbor and can find no mention of it. The report references "the river" as well. The only river close enough to Cold harbor worth mention is The Chickahominy and at that point it is mostly swampy bottom land and a few wide but shallow sections that are navigable only by a canoe. I live about eight miles from the battlefield. There were some short rail spurs in the area but The Virginia Central Railroad and The Richmond/York River Railroad are some distance from Cold Harbor. This report was obviously made during the retreat from Cold Harbor and approach to Petersburg. The Dictator was a thirteen inch diameter mortar that was mounted on a large wooden platform and was fired from a fixed position mostly on the civilian inhabitants of Petersburg during the seige. There are pictures of it on a rail car but I doubt nor have I read any evidence of it being fired from a temporary position while being transported around "the battlefield". There is a picture of a rail car mounted gun that appears to be a large Parrott or Columbiad fixed behind a sloping armor covered timber shield. It gave the appearance of a large "cow cather" on the front of an engine. I believe this particular photo is a Federal gun but from my study the Confederates were the first to attempt this rail mounted artillery. Just as they can be credited with the first aircraft carrier, a barge from which a hot air ballon was launched, and land mines, which were improvised trip wire detonated artillery shells. Southerners were very ingenious in war. Remember the Hunley had recessed rivets on it to help streamline it in the water as well as a crude battery and electric motor. The motor would only sustain a speed of three knots and the current in Charleston Harbor ran at five knots therefore rendering the motor ineffective.
I'd say war not neccesity is the mother of invention. haha.
New Kent County, VA