BOSTON -- Gov. Mitt Romney reassured residents of Boston Thursday about the safety of the city's transit system after explosions rocked London for the second time in two weeks.

Romney put Massachusetts transportation authorities on Orange Plus Alert after the first London attacks on July 7, which left more than 50 people dead.

Four explosions went off Thursday at three subway stations and on a bus. So far, there have been no reports of serious injuries, but the second attack raised more security concerns for local rail systems.

Romney was in Washington, D.C., to talk to federal officials about his health care plan, but returned to Boston early to address residents' concerns about the latest incident in London.

"(Massachusetts Bay Transportation Authority General Manager) Dan Grabauskas reminded me that we are at Orange Alert, but that we have moved to what we are calling Orange Plus. It means we have additional personnel at the stations making sure that all of our facilities are closed at night and sealed off so that no one can get to the equipment. We are adding security. We are adding communications to riders," Romney said.

Romney planned to board an MBTA subway to demonstrate the safety of the system in the commonwealth.

"We will be riding it briefly and taking a ride down the Red Line today -- not just to prove that it is safe, which I think everybody already knows, but also to say, 'Thank you' to the men and women of the T organization for their work," Romney said.

Massachusetts Secretary of Public Safety Edward Flynn said that officials have been in constant communication with the Department of Homeland Security and U.S. Attorney Michael Sullivan to keep on top of the situation in London.

"All of our federal partners have assured us that even today, there is no credible threat against the United States on its homeland soil or any current credible threat against our mass transit systems," Flynn said.

Grabauskas said that there have been no disruptions in service on the MBTA Thursday, and urged riders to alert officials to suspicious activity.

"We have deployed, at this point, additional personnel, if you want to think of it as Orange Alert plus, at this point. We stepped up some of our in-station announcements and bus announcements. We deployed additional supervisory personal over and above what is mandated by the alert -- all in attempt to be prepared, to be vigilant. We talked to our customers, who throughout this process, we hope that if they see something, they will say something to a station personnel," he said.

Commuters were responding to the "See Something, Say Something" campaign, because within the two weeks since the first London bombings, the agency has had numerous reports of bags left unattended.

Travelers on the MBTA's Green Line were greeted with the intercom messages about the agency's safety campaign.

"They are making more announcements, telling people to be more alert. I haven't really seen a security presence, but there is a lot more awareness among the riders," Somerville, Mass., rider Ellen Twaddell said.