My experience: I take the train on a daily basis because I do not have a license and can direct you to almost anywhere if I've got a map and two minutes. Here's a couple things to know about the T now, as of May 26, 2022:
- UPDATE: As of late April 2022, masks are now optional on the MBTA. Please know it is highly recommended to wear a mask.
- The Green Line, which is the main line used to get to the Hynes, is currently under construction. The Hynes and Prudential stops are not impacted. As of March 2022, trains now head out to Lechmere and to the new stop, Union Square. Train end-destinations are: B and C trains terminate at Government Center, D trains terminate at North Station, and E trains at Union Square.
- They have updated the tickets and ticket machines, which may not play nice with older Charlie cards. And, as @Lamron pointed out below, your Charlie card might now be expired and you will have to get a ticket or a new card when coming into town. If you had money on it, you will have to go to Downtown Crossing to get that transferred. For getting a new card and putting your old money on it: the Downtown Crossing Charlie Card store is only open Wednesday and Friday for the general public, 12:30pm to 5pm. Tickets can still be purchased at any machine.
- UPDATE: @Elli21486 described it best below: if your old Charlie ticket has a BLACK strip, it has to be inserted at gates with a rectangular tap pad. If your new Charlie ticket has a GREY strip, it's tap and must be used at gates with a circle tap pad. If you get stuck, there are MBTA staff in red coats hanging around stations who can assist.
For the Green line: You can either get out at the Hynes stop (B, C, D trains), and walk to the Hynes by turning left and crossing the bridge toward Berklee College of Music. Look left and you will see the Hynes and Prudential buildings, and walk there.
OR, the indoor-only method: Take the E to "Prudential" instead of any other train to the Hynes stop. It's a whole walk indoors through a nice mall, instead of a walk outside along a major road with a big road crossing involved. (And the walk through the air conditioned mall will much better in the potentially hot/rainy late May weather!)
For the orange line, some lovely people below have pointed out alternative ways to get there.
If you took the Orange Line Train In: When you get off at Back Bay station, there are some exit gates behind the stairs. Through these, you can go through a tunnel under Dartmouth Street to get to the Copley Place Mall. You walk through the mall towards the Mariott, and then to the Skybridge into the Prudential. (This spits you out near the CVS.) Through there, follow the signs to get to the Hynes and Sheraton.
If you took the Commuter Rail/Purple Line Train In: You can either locate the stairs that take you down to the tunnel and behind the fare gates, or step outside for a bit to cross Dartmouth Street and walk into the Copley Place Mall. All indoors from there.
Thank you @PatrickD, @Elli21486, and @Shirley Dulcey. Their posts below go into a bit more detail, especially Elli's and Patrick's with precise instructions.
MBTA-ers, add on your tips about close to transit hotels, last-train out advice, and such, and those with questions: ask away!