The Massachusetts State Police is indeed trunked. The area you'll be listening to will require two data channels:
MSP Zone 3: 859.8375
MSP Zone 4: 860.2375 (Cape Cod)
These very rarely (only when techs are working on the system) change. If you're coming to Cape Cod you won't be on I-95 in MA if you're coming from the west. You'll pick up I-195 east out of Providence, and the two aforementioned frequencies should do the trick as control channel frequencies all the way. The troop you will hear will be Troop D. The barracks you'll hear are D3 (Dartmouth), D7 (Bourne), and D2 (Yarmouth). The two MSP talkgroups you'll want to monitor primarily will be 34000 and 34032. However, monitoring the MSP goes far beyond that. My website will lend some good information for the Cape Cod area, but the best site for MSP info statewide can be found at:
http://lynx.dac.neu.edu/s/stjohnso/ematrunk/index.html
You'll find that most Cape Cod police departments can be heard on the MSP Zone 04 system. You'll hear all FD's there. There are six Cape Cod PD's that remain on conventional 800:
Bourne: 851.1625
Sandwich: 851.3875
Falmouth: 855.4625
Mashpee: 851.3125
Barnstable: 855.2125
Yarmouth: 855.2375
All of these departments use a Digital Coded Squelch tone of 445.
You'll also find Hyannis Fire on 868.2125 (203.5 PL). Check
http://www.scancapecod.net for a slew of information for the cape. If I can help further please feel free to e-mail me, feel free to register and ask questions at Scan Cape Cod, or I'll respond here.