so apparently it wasn't pranksters but the Cartoon Network who planted the "bombs" around Boston.
It was an add campaign for Adult Swim. Funny bastards! And the train ride home was fine. It actually was less crowded than normal. They must have scared people into the bars.

Here is their statement from Shirley Powell of TBS Corporate Communications:

Turner Broadcasting System, Inc. Chairman and CEO Phil Kent, commenting on the incident today, said, "We apologize to the citizens of Boston that part of a marketing campaign was mistaken for a public danger.

"We appreciate the gravity of this situation and, like any responsible company would, are putting all necessary resources toward understanding the facts surrounding it as quickly as possible.

"As soon as we realized that an element of the campaign was being mistaken for something potentially dangerous, appropriate law enforcement officials were notified and through Federal law enforcement channels, we identified the specific locations of the advertisements in all 10 cities in which they are posted.

"We also directed the third-party marketing firm who posted the advertisements to take them down immediately.

"We appreciate the commitment demonstrated by the Boston police department and other law enforcement agencies, as well as the Massachusetts Governor's Office, and deeply regret the hardships experienced as a result of this incident."