I think Sundowner hit it on the head: The CSX units are Dash 9 internals in a Dash 8 body. In fact, they were originally listed by CSX as CW44-8's and were reclassified later on.
Here's a shot of the class unit, CSX # 9000: CSX 9000 All of the CSX units have the standard Dash 8 trucks.
A little late to the party... but the thicker radiator wings on the -8s (and -9s) denote that they have split-cooling, whereby the turbo and air compressor cooling circuits are separate from the prime mover cooling circuit. It is a phase feature for the -8 line...
NS ordered it's -9s and ES40DCs at 4000hp because long ago, they found that 4000hp per unit was the optimal power needed for any train systemwide (note this was before the CR split). Since the tractive effort of the units is the same regardless of power output, keeping them at 4000hp reduces fuel costs, yet still allows any train to be run, albiet at a slightly lower top speed, but given NS' terrian, top speed really wasn't an issue. There is also some 'wear and tear' savings, but in all honesy it's pretty minimal, if anything it slightly lengthens the time between overhuals... but there really isn't any actual cost savings, it costs the some to R&R a power assembly or injector assemby after warranty regardless. There was no derating of the warranty either.
