Well, that is a tough question. They were designed for different uses. The DDA40X was used in fast freight and was geared for speed. The SD90AC was designed for lugging. The DDA40X has two more axles than the SD90AC. The DD has the old style slip control, aka an engineer. The SD90AC has computer slip control. Who would win, the DD would lose in the end to the SD90AC in the tug of war contest. Who would win the speed race? The DD would win hands down. Having run both in my short railroad career, the DD was more responsive in all ways. It would get up and run away in speed and might even win for a short while in a tug of war. The SD90AC with its AC traction motors, I think would eventually win. Now for a third type of contest, how about a tracker pull? With the right engineer, the DD would hold up pretty darn good.
Greg Elems
