Is this an AC or DC model?

I don’t have one of these but from an old listing on the Graham’s Trains site (a good source for archived product info) and some old eBay listings, this appears to be Lionel model 6-18910. The replacement parts listing on Lionelsupport.com shows that it is conventional only with an E-unit (as illustrated below). Clearly a DC motor but as it has center rail collectors and an E-unit, I would have thought it was made to run on AC powered track.

That's helpful; it says the same thing on the replacement parts page.

I'm not familiar with any Lionel O gauge engines that were designed to run on AC or DC power, whether modern or older like this one. I assume that a DC transformer could simply be connected via a standard 3rail track lock-on and whichever of + or - was supplied to the center rail would be fed to the E-unit and then to the DC motor as DC power. I also suppose that depending on polarity the engine might start out running forwards or backwards and the direction could be changed via the E-unit. But I'm just guessing about this detail. 

P.S. I suppose that the answer to the thread title - is this an AC or DC model? - is yes, as in it's both.