For the second time in two months, a pipeline that supplies gasoline to millions of people was shut down, raising the specter of another round of gas shortages and price increases. The disruption occurred when a track hoe - a machine used to remove dirt - struck the pipeline, ignited gasoline and caused an explosion Monday that sent flames and thick black smoke soaring over a forest in northern Alabama, Colonial Pipeline said. One worker was killed and a half-dozen were injured.