BOSTON (REUTERS) - Massachusetts officials called out National Guard troops on Tuesday to help dig out from a trio of massive snowstorms in the past two weeks, noting that at least seven roof collapses had been reported as a result of the heavy accumulation.

The head of the Massachusetts Bay Transportation Authority defended the decision to shut Boston-area subways and rail service as schools remained closed across much of the area.

Some 500 National Guard troops were to deploy across eastern Massachusetts to help clear the roughly 1.8 metres of snow that has fallen in the past two weeks, Massachusetts Governor Charlie Baker told reporters.

"We need to get the Commonwealth back to work and we need to get our kids back to school," Baker told reporters, adding that his counterparts in New York and Vermont had agreed to lend heavy equipment to help with snow removal.

Baker also said he had approved the purchase of two snow-melting machines capable of melting 120 tons of snow per hour.

The city's rail system will reopen on a limited schedule on Wednesday, and public schools will hold classes, Boston Mayor Marty Walsh told reporters.

"We've been informed that the MBTA will be operating tomorrow on a limited schedule, as of right now," Walsh said.

Beverly Scott, general manager of the MBTA, said she had no choice on Monday afternoon but to order all rail services shut down, an announcement that came right before the evening rush hour and a few hours before the shutdown.

"We do not control Mother Nature," Scott told reporters, visibly angry about criticism of her handling of the storm.

The train system carries about 950,000 riders on a typical weekday.

Heavy snow on tracks had caused trains to lose contact with the electricity-carrying third rail across the system's 1,290 kilometres of tracks, prompting the shutdown, Scott said.

This has been the area's snowiest 30-day period on record. Much of eastern Massachusetts has received more than 6 feet of snow so far this year, making for the ninth snowiest winter on record.

At least seven roof collapses were blamed on the storm, including a historic building in Hingham, Massachusetts, south of Boston. Officials urged owners of buildings with flat roofs to clear them of snow.

"These roofs are getting heavier and heavier," Walsh said."You don't want a situation with your roof collapsing."

Walsh said that two school holidays later in the year had been cancelled to make up for lost days and that he was in negotiation with the city's teachers' union to allow for any additional snow days.

The repeated snow days were taking a toll on parents. Kelsey Wirth, 45, a climate change activist who works out of her home, said she was running out of ideas to occupy her 8- and 9-year-old daughters.

"Work never ends. Even if Boston shuts down, life goes on," Wirth said.

"When I heard school was closed again, I said, 'OK, today is project day,'" referring to lining up chores for her children.