I worked part-time for the Post Office while attending BC during the late 60s/early 70s. Every now and then I would deliver one of the mail routes in Newton Lower Falls.
As I recall the branch ran behind and parallel to Pine Grove Avenue once the 128/95 crossing was complete. "Behind" in the sense the line ran through-or close to-several backyards on Pine Grove before eventually crossing Pine Grove. From that point the line ran between St. Mary's St. and Baker Place, crossed Concord St., entered property abutted by and housing (then) the Bigelow Oil Co. offices and terminal, crossed the Charles River on a wood trestle, entered Wellesley (Grossman's Lumber) and did, in fact, cross Route 16 for a very short, i.e. >30 yards, distance, basically room enough to park their engine. B&A/PC crews would leave their engine and walk the short distance for coffee at the former Pete's Coffee Shop.
Freight consisted of lumber for Grossman's (boxcars and various lumber transporters) and (rarely) a tank car left at Bigelow Oil on a very short spur track. As noted previously, there had to be a run around track on Grossman property to facilitate switching/exchange of MTs with loads.