If you spent the last gazillion years swerving around cranes to get to the North End because of the Big Dig, you would. Or watching the toll on 90 double so you can get into town.
As anyone who lives in Boston can tell you, it's the T (and the roads in general) are in a constant state of flux. Roads close, open, and become one way going the other way with little or no warning. Add a population of aggressive and generally impatient drivers (once I adjusted to driving in Boston, I realized I couldn't drive anywhere else...my fiance was mortified when she took me to Pittsburgh). In other words, it's the perfect place for a Rigger.
On to the T: Most of the T is below ground, although there are stretches of it running aboveground. I don't know it end-to-end, but here's a start:
On the Red Line, Charles/Mass General Hospital is above around and so is the bridge heading over to Cambridge. It goes underground quickly after that and remains there out to Alewife.
On the Orange Line there are spots of aboveground travel between NE Medical and Forest Hills, although most of the stations are still underground.
Bowdoin (end of the Blue Line) isn't always open. Yes, there's some schedual to it. No one I know can tell you what it is.
As a rule of thumb, most of the center stops and tracks are below line while it comes aboveground in the 'burbs.
Commonwealth Ave offers a great place for a car chase. Two lanes going either way divided by an aboveground T line (Green) with occasional chances to zip into opposing traffic.
The Silver Line doesn't exist.
There are a lot of different levels and old tunnels out there. Just riding through Park St. you'll see disused cars and tracks in the flickering light before you pull up to the platform. If I was Mama, I'd be sure to control the rectangle (trapezoid) formed by Chinatown, Aquarium, North Station, and Park. Lots of old build there and access to the vital parts of the city.
And, afaik, most if not all of the underground tracks run off a third rail.