FYI, if you don't already have a CharlieCard, you no longer need to get a new one to pay for the T.
You can tap your phone (Apple, Google, Samsung Pay) or tap a credit card on bright white screens while entering any subway station, street-level Green Line trolley, or bus, just like you already do in NYC. You will enjoy the same transfers that you would with a CharlieCard.
Here is another video on the tap-to-pay roll out.
If you still have a CharlieCard and want to clean out the balance, of course you can still use it. Just tap the older CharlieCard readers with CharlieCard stickers on them (these are separate from the white screens mentioned above).
Also, if you plan on using the T a lot, you can still load a CharlieCard with a 7-day pass, or buy a 7-day paper CharlieTicket from any subway station vending machine. You would need to use the T
at least 10 times during your time in Boston to make a 7-day pass worth it.
The new tap-to-pay feature mentioned above does not incorporate passes or automatic NYC/London-style fare-capping yet.
This having been said, you might want to keep your phones, wallets, purses, or payment-enabled smart watches away from these white screens while entering a station or bus, or while taking a Green Line trolley so you don't inadvertently pay extra.
If you use the commuter rail, you will need the separate mTicket app.
The T has had a rough few years but things have improved dramatically in the last 12 months. But it's an old system and problems may arise in whack-a-mole fashion.
You may want to check the MBTA website to see if there are any weekend or nighttime alerts or diversions.
Editing again to add: if you're traveling light, you can jump on the Silver Line at the airport for
free when you arrive. You can then transfer to the subway for free at South Station. The Silver Line is a set of rapid bus lines; the one that serves the airport has dedicated baggage racks. It is not otherwise free if you're not jumping on it at the airport.
And another tip: use general street smarts while out and about, and be especially careful with your wallets, purses, and belongings when entering or exiting a fare gate. These gates usually stay open just long enough to allow fare evaders to scoot in behind you.