Another wrinkle is the recent revelation (via MaineDOT RFPs) that several bridges on the Rockland Branch are in much worse condition than many of us realized. Running a Budd RDC (120,000 lbs) over those bridges is one thing; running a fully-fueled P42DC (268,000 lbs) over them is another.
I'm not an expert on bridge ratings, but maybe you can run an RDC over bridges in that condition at 40 MPH, in contrast to the 10 or 5 MPH slow orders that would likely be necessary for operating a P42 over them. I suspect neither Amtrak nor the traveling public would be in favor of the latter.
The bridges will eventually be repaired, and I suspect Amtrak will operate on the branch eventually, but there's probably no harm in having Finger Lakes operate an RDC-based service as an interim measure.
Edit: this is what those AEAX RDCs look like inside, in case anyone is curious:
http://rrpicturearchives.net/showPictur ... id=2108266