Add the Mattapan line

[idreyn]

A user wrote in to point out that the ancient PCC cars are being retrofitted with new components, which may be of interest to the same folks who are interested in hunting down new trains. New or not, this will be a good addition as fleet delivery wears on and the NTT becomes just the "train tracker" with Blue, Silver, etc.

We could possibly tack this onto the bottom of the Ashmont strip map, though that's technically harder (and maybe not as appropriate for the use case) than just putting it into its own tab.

[nathan-weinberg]

Great idea!