REVERE – Police said a middle-aged machine operator suffered minor injuries after an excavator fell through a bridge that was being repaired and onto nearby train tracks.The accident happened on the Route 145 bridge at the intersection of Route 16 and Winthrop Avenue Saturday at about 6:25 a.m.According to Lt. David Callahan, who was at the scene, the 50-year-old West Bridgewater man, employed at Atlantic Demolition, suffered non-life-threatening injuries.?He had minor cuts and lacerations to his head from the fall,” Callahan said. “He was transported to Massachusetts General Hospital. But he was able to get out of the Caterpillar, a 345c excavator, on his own, no tools required. The cage protected him.”Callahan said fellow Atlantic Demolition workers, along with fire department and Cataldo Ambulance personnel, helped the operator to escape. Callahan said the victim was trapped for five to 10 minutes.MassDOT officials said they did not expect the accident to affect the bridge?s re-opening today for commuters. The bridge was stabilized, and the crane was removed by late morning Saturday.Repairs will continue on the bridge next weekend.