Axle loads of the SD-45's are less than most of the 4-axle power that has been used on the line.
Long wheelbase of the six axle truck is the main issue. BN ran LMX (heaviest axle load of BN power, BW40-8 heavier still (BNSF 500 series--also have been out CW line)). LMX and BN B-30-7AB ran out CW and those floating bolster trucks GE uses on it's 4-axle units are just as hard on track as 6-axle EMD trucks, maybe more so. No matter what they run, light rail and rotten ties are the big issue they face. Loaded unit trains are just plain hard on track no matter what pulls them. If the line is to last much longer, big time track work is needed now.