When the low-clearing East Street MBTA commuter rail bridge -- the site of several accidents per year over the last several years -- tore off the roof of a bus on Saturday afternoon, it marked the first time one of the T's own vehicle failed to pass under the bridge, according to MBTA and Westwood officials.

Neither Westwood Fire Department captain Richard Cerrullo or MBTA spokeswoman Lydia Rivera could recall an incident involving an MBTA bus at the bridge. In 2011, trucks carrying wine, cookies and an air conditioning unit all failed to pass under the bridge, which is very clearly marked with the words "Low Clearance."

According to Cerrullo, the bus was not on its regular route, as it was supposed to be making shuttle runs between commuter rail stops. According to the Boston Herald, the driver was lost and well off-course, and was supposed to be shuttling between the Readville and Dedham stations.

The bus, at the time of the accident, was unoccupied Rivera said.

Rivera said the MBTA and transit police would be investigating the incident. Rivera could not be reached for further comment to confirm that any investigation would look into why the bus attempted to clear the bridge in the first place, or how the driver had become so off-course.

Westwood Police were on the scene Saturday to assist Transit Police, but referred questions about the incident to the MBTA and the fire department.

Cerrullo told Patch that the decision to evacuate Roche Bros., the Westwood Little League fields and several nearby houses was precautionary and done in response to the leaking of the compressed natural gas (CNG) that fueled the bus. "The wind was drifting that way," he said.

In the case of Roche Bros., Cerrullo said, the response team was concerned that the CNG, which rises quickly, may have gotten into the store's ventilation system. Upon confirming that it hadn't about an hour and a half later, he said, the store was reopened.

Cerrullo said that a hazardous materials team was not brought on to the Saturday afternoon accident scene.

In October, Westwood officials said any effort to make the bridge any higher would have to start with the MBTA. Asked whether this is something the MBTA had considered or talked about recently, Rivera replied, "Not to my knowledge."