All Amtrak's current diesels have DC traction motors. Amtrak doesn't need the low speed heavy pull capacity that freight engines do that AC traction motors provide. They did have a some AC units in the 90s that were kind of testing the concept for passenger use. I don't recall the actual model designation, but the railfan slang name for them was "winnebagos" because the front end kinda of looked like an RV's. I am pretty sure they were EMD products.
I don't know the costs involved but speculate there is a price difference that at least partly drove the decision on DC versus AC traction motor.
