See "The Military Operations of General Beauregard" by Alfred Roman, who spent most of the CSA's existence as Beauregards' Aide De Camp.
I don't know where you got the rail car mounted gun. There was a ten mile lapse of rail connection that necessetated the troops moved from the north marching the balance of distance on foot. Attempts were made to get rail line from farther south to bridge the gap, but the owner of the road wouldn't part with it.
As you've noticed, the battle didn't proceed as planned. Beauregard was back towards his headquarters in Charleston, S.C. when conflict occured. If he'd have stuck around the field he might have followed up on the event.
If you find any Gamble references to artillery in this engagement please inform me.