I would keep an eye on some of Pan Am/ST's many licensed but unused frequencies, such as:
160.230 licensed for fixed base, repeater (!) and mobiles systemwide
160.260 licensed for fixed base and mobiles in former B&M territory
160.350 (old East Deerfield hump channel) licensed for fixed base and mobiles in former B&M territory
160.380 (old MEC MOW) licensed for FB and mobiles systemwide
160.440 licensed for FB and mobiles systemwide
161.235 (old B&M Police repeater) licensed for FB in former B&M territory
161.310 (old Northside commuter rail) licensed for FB and mobiles systemwide
161.370 (B&M MOW lightly used) FB and mobiles systemwide
161.520; yes, this is the mobile side of the B&M road channel (and has been since the 1970s as I understand it), BUT it is licensed for FB all over.
Remember that CSX is keeping the ST train dispatchers at North Billerica (there is a brand-new joint Keolis/ST dispatch center nearing completion there); there are no plans on moving them to Jacksonville or anywhere else. Another thing to remember is that the MBTA owns the Western Route all the way to the MA/NH state line, and the Fitchburg all the way to Westminster. It's not out of the realm of possibility that future CSX and B&E have Keolis take over dispatching of those areas; Keolis already has 160.965 earmarked for use on the Fitchburg in the event they split it from the current Boston West frequency of 160.590.
Speculation/opinion:
I would imagine that B&E would use one of the above mentioned, presently-licensed frequencies for use west of Fitchburg/Westminster, instead of attempting to get a new freq. coordinated and licensed. Also keep in mind that B&E has no announced start-up date.
My best guess is that CSX will keep the current frequencies in use for the time being. If they ever decide to change to their style of separate "Road/Dispatcher" frequencies they have plenty of currently coordinated and licensed frequencies to choose from to pair with what is already in use, especially in Maine where many RR frequencies are used by MSCOMMNET.