By the Dean Street branch, I assume you mean the Bay Colony's
Dean Street Industrial Track that starts at Weir Jct in Taunton.
Speed is 10MPH.
As for the mainline (and disregarding the lack of a current CSX
Temporary speed summary for reference):
Attleboro to Whittenton Jct is 30 for both passenger and freight.
From Whittenton to Oak Street is 20MPH
Through downtown Taunton is only 10MPH (all the crossings)
with a bump to 15MPH just before Weir Jct.
Weir Jct to Cotely Jct is 25MPH
Cotely Jct is 10MPH.
It's 10MPH to New Bedford and Fall River.