Your train would be flying on level grade. Many trains running up Tehachapi only get 2.8 to 3.2 HPT and then can make 12-15 MPH on a 2.8% or so uphill grade.

Also what motive power is on your train wouldn't really matter as long as the HPT is the same. 2 SD70ACe or 3 SD40-2 would be about the same. The difference in motive power (AC vs DC) would be in startup ability (how quickly to acclerate to say 20 MPH), adhesion on heavy grades and very importantly if the speed was below that critical 9-12 MPH for DC then of course DC power would have to be time limited while your AC engines could pull all night. Once saw a loaded coal train in Utah (SP) with a AC4400CW, MK5000C and a Dash 8-44CW. The Dash 8 was already off-line and the MK died on the uphill climb. The AC44 pulled that train over the summit at 1 MPH and did no damage. A DC unit would be all melted wires and birdnested traction motors.