The Silver Line
's name is somewhat ambiguous because it's a bus line that masquerades as a subway line. The website (allaboutsilverline.com) is a little bit dodgy in describing what the Silver Line actually is
; they prefer to say that it "combines subway and bus technology." Thanks, guys.
The Silver Line, really, is an accordion-style bus that runs through "exclusive right-of-ways" in Boston. This means bus lanes, but it also means bus-dedicated tunnels; that's the cool part. They haven't implemented that yet.
There are three phases of Silver Line service. Phase I has been implemented, a bus lane down Washington Street from Downtown to Dudley Square by way of Roxbury and the South End.
Phase II is currently being built, and will run in a one-mile underground corridor from South Station to the World Trade Center, and onward to Logan or the Convention Center.
Phase III is another tunnel connecting the first two.
Washington St. Service (implemented)
Phase II Service (under construction)
The buses now emerge from underground and split for:
Phase III Service (still being designed)
Sources: mbta.com and allaboutsilverline.com