A power outage plunged Alewife Station into darkness Monday night and trapped one person in an elevator, an MBTA spokeswoman said. The outage began at about 9:45 p.m. and ended shortly after 11 p.m., MBTA spokeswoman Kelly Smith said. Power still flowed to the third rail, so trains continued to run, she said, but one remained at the platform to provide light, reducing service to a single track. Smith said that Transit Police were on scene and that sufficient light was available for passengers to move about safely. No injuries were reported. Captain Mark Cunningham of the Cambridge Fire Department said the person trapped was freed quickly. The cause of the outage was not immediately known.