It also depends on how CSX decides each line is classified. A bunch of Western Maryland lines for instance branching out of Hagerstown, MD were basically secondary or branch line status, even though they saw regular heavy freights (2-4 trains a day in some instances) and were good for 10MPH or 25MPH. Even the line from Hagerstown to Chambersburg after they built the large intermodal terminal there in 2009 was 10MPH for most of the journey hosting daily pairs of long intermodals and local manifest traffic. The Gettysburg line which was secondary status always interested me as it saw regular heavy freights but was really no better than Pan Am's existing D1, in fact it was basically just old relay jointed rail.
I guess the good thing here is that D1 will probably get designated a primary mainline as far as Waterville, since there are no potential secondary routes, but east of there, I could see it being a branch status until CSX decides there is something worthwhile (or even then maybe they will make it a solid 10 or 25). Only time will tell, but I don't expect there to be a large push for 40MPH past Waterville in the first year or two like we saw with CP taking over CMQ, since the business pressures seem to be a bit different. There is a CSX earnings call this week upcoming, so hopefully some more light is shed as to where things are presently (I'm certain at least one investor will ask)