DC/AC and the number of rails are not connected. A DC loco could use a 3-rail system - those old outside third rail locos were often DC - and a 2-rail system could certainly be run on AC power. It has to do with the motor being set up for AC or DC, not the route of the current.
BTW - Right-of-Way made a 3-rail version of this B&O so-called "Dockside" back in the 80's. Well-done, runs smoothly, nicely detailed. I see them on eBay for $200 - $300. I like projects, but if you can buy it ready to go...