Complex sentence: North Street connects to Boston-Providence Turnpike, which carries U.S.1 (and was never a turnpike); this road merges into the old route of Washington Street south of the railroad.
Split into simpler sentences: North Street connects to the Boston-Providence Turnpike, which carries US Route 1 (and was never actually a turnpike, a toll road). This road merges into the old path of Washington Street south of the railroad.