Around 8:50 p.m., Boston firefighters and Transit Police rushed to the area of Arlington and Boylston streets after several people reported hearing a loud explosion. Cara Terez, sitting on the Rattlesnake roof deck, tweeted:
Loud LOUD explosion.
As a precaution, the T shut Green Line service as firefighters looked around on the street and in the station for the source of the boom, but they found nothing, an inbound Green Line driver reported the tunnel between Copley and Arlington was fine and service started up about 10 minutes later.
Brian D'Amico tweets police speculate a tour bus may have had an engine backfire or two.