The graffiti-covered railroad span that threads diagonally under the Boston University Bridge will be closed to most train traffic for two weeks, after an inspection found some of its crossbeams need to be replaced.
Many people are unaware that the 90-year-old railroad bridge across the Charles River is in use at all, but it is a critical piece of transportation infrastructure. It is part of the little known Grand Junction Railroad, which starts in Brighton and wends its way through Cambridge, Somerville, Everett, and Chelsea, connecting train lines that go to North Station with the ones that lead to South Station.
The freight line’s tracks are also used by Amtrak and the Massachusetts Bay Transportation Authority to shift engines and coaches for maintenance.
The repair work has begun, according to MBTA spokesman Joe Pesaturo. In the meantime, the train systems that use the bridge have come up with alternative plans to ensure no interruptions in service.
Two weeks ago, a visual inspection of the bridge found signs of movement in its trusses, according to Scott Farmelant, spokesman for the Massachusetts Bay Commuter Railroad Co., a contractor that operates the MBTA’s commuter rail service. A bridge engineering inspection firm was brought in, and on Wednesday, Farmelant said, the bridge was closed to freight cars and locomotives. He emphasized there is no public safety threat, since passenger trains do not use the bridge, and said the closure will not affect commuter rail services.
Pesaturo said in an e-mail that seven of the bridge’s 44 crossbeams need to be replaced. Repairs are expected to take about two weeks, and the cost of the project had not been finalized.
In preparation for the bridge’s closing, Pesaturo said, a number of organizations and companies have worked together to devise solutions.
Amtrak, for example, will do routine maintenance on its Downeaster trains, which provide service to New Hampshire and Maine, at the MBTA’s facility in Somerville, and the commuter rail company will perform maintenance on some of its equipment at the Amtrak facility in South Boston.
Cliff Cole, a spokesman for Amtrak, said the bridge is used to transport locomotive engines to Amtrak’s maintenance yard, and the repair project will have no impact on Downeaster schedules.
“Amtrak has been working around the closure by maneuvering the equipment through different detour routes,” and pursuing an agreement to have equipment maintained at different sites, Cole said.
Pan Am Railways and CSX Corp. reached an agreement to “transfer freight rail trains in the Worcester area,” Pesaturo wrote.
The bridge is not completely closed; empty commuter rail coaches can still be transferred over the bridge.
But in an illustration of just how important the bridge is, Farmelant said, two commuter rail locomotives would be transferred this weekend to the MBTA’s Somerville repair facility by taking a roundabout route through the Worcester area.
The transfer, which would normally involve crossing the Charles on the BU span, will utilize tracks that go through Ayer and then back east on the Fitchburg commuter rail line to Somerville.
“The system ultimately is wholly dependent on aging infrastructure,” Farmelant said. “This shows how critically important these investments the governor and others keep talking about into our infrastructure are.”Carolyn Y. Johnson can be reached at cjohnson@ globe.com. Follow her on Twitter @carolynyjohnson.