That hill going through the rock cut between the retaining walls up to Park Ave is much too steep and narrow for any rail line.That hill used to be a lot higher before they widened Route 2, and had a spectacular view of Boston. I was kind of sad when they lowered and widened the highway because the view wasn't as good.
When I was a little kid, streetcars were still running on Mass Ave in Cambridge and Arlington, with Type 4 or 5 cars running from the North Cambridge carbarn, and PCC's running from Arlington Heights. I used to live near Powder House Square in Somerville, and we'd take what is now the 96 bus, which at that point ran from Medford Square to a dirt lot turnaround on Mass Ave near Saint John's Church. We'd get off the bus and cross over to the streetcar platform in the middle of Mass Ave. I was only 5 at the time, and I couldn't understand why the "nice" PCC streetcars passed us by, and only the rickety old Type 4/5 cars would stop for us! Arlington Center used to be quite busy with multiple streetcar tracks in the middle of Mass Ave, the B&M trains crossing several times a day, and I think the Route 80 bus from Lechmere was a trackless trolley at that point. So there were a lot of wires and a lot of track. I remember that when a B&M train had to cross Mass Ave to get to the Arlington station, a trainman would climb down from the train and activate a switch that would turn all the traffic lights red on Mass Ave and all the adjoining streets. Very colorful!