Emergency crews battled a fire at an MBTA maintenance facility in Somerville Tuesday morning. The fire started around 6 a.m. at the Commuter Rail Maintenance Facility and was quickly brought under control. The Cambridge Fire Department said the fire originated in a train car before spreading to the roof of the building. A spokesperson for Keolis, which operates the Commuter Rail, told Boston 25 there was a "minor fire in the exhaust ventilation of the building."