Dozens of residents were evacuated, services on a rail line were halted, and roads around a school were closed to traffic after a workman using an excavator ruptured a main gas line.  The worker was part of a crew for a plumbing company. Reports indicate he was aware the gas line was in the vicinity of the dig, but clipped the side with his excavator.  A police official said the proximity of the gas leak to the train line was a major concern for emergency services, and that explosive gas concentrations were detected along the rail line.  About two hours later residents were allowed back and the train service resumed.   13/7/2004