The trucks on the trailing unit are, as you say, like those on Dash-9 and on many AC44. This truck design has higher adhesion (less weight transfer) characteristics than the truck used on U-, Dash-7 and Dash-8 series models. It was introduced on the C44-9W in 1993, and can be used with either DC or AC motors.
It is NOT a "radial" truck: the axles are in fixed parallel positions. EMD introduced a radial truck, in which the axles can rotate independently enough so that, when going around a curve, each axle stays perpendicular to the track (and so parallel to the radius of the curve): obviously better for track and flange wear. GE didn't have a radial truck design ready when the Dash-9 and AC44 were introduced, but developed one shortly thereafter (it was retrofitted to an early CSX AC44 for testing in-- I think-- 1995). CSX liked it, and specified it for their later orders (AC44 numbered 200 and up). Canadian Pacific likes it: after getting the earlier truck on their first AC44, they have gotten later orders with radial trucks. KCS and the Mexican railroads have radial trucks on their AC44. UP tested GE's radial truck on a small number of AC44, and seems to have decided that it isn't worth the extra (?) cost and maintenance hassle: almost all their AC44 have the "Dash-9" style truck. BNSF, after getting a huge fleet of Dash-9, has started buying GE AC locomotives with -- like UP's units-- the Dash-9 truck.
I don't know why the different railroads have reacted this way. Maybe CP has worse grade/curve combinations than the western U.S. railroads, and CSX evidently wants to maximize tractive effort, often at low speeds. This **SUGGESTS** that the radial truck offers advantages in some applications and less of an advantage in others, but I don't know anything concrete.
In principle, I think GE's radial truck could be used with DC traction motors, but as far as I know no Dash-9 have been built with radial trucks.