Almost all locomotives on the NS roster are DC locomotives. The only AC locomotives on the roster came from Conrail. Even the engines NS orders today (ES40DC and SD70M-2) are DC traction.
The general category, "DC locomotives" is not specific enough here. Original nomenclature termed units as being "DC" because they had DC main generators, as opposed to the more modern alternator/rectifier units. In those days, all
diesel-electric locomotive traction motors were DC.
EMD began using alternator/rectifier main generators on their 40 series units, in the mid-1960s, in order to overcome the problems associated with DC generators producing high amounts of current. GE began to use them on their higher horsepower units in the same timeframe. GE U23B/C units were among the last "DC" units, as termed in that time period. EMD GP/SD38s, ditto.
Indeed, the majority of today's NS fleet is equipped with DC traction motors. Their main generators, however, are predominently 3-phase AC, rectified to DC for traction.
I hope I've made that clear, anyway.