Somewhere around Bela Vista Station on Portugal's only subway system one day last week, an incident caused officials to switch off the power, leaving trains full of passengers stranded for more than an hour. No, there hadn't been an accident or mechanical failure , and there was no threat of terrorism. The problem also had nothing to do with the construction that is extending the Red Line so it will cross the Green and Yellow and connect with the Blue sometime next year, extending the Lisbon Metro to 25 miles. In fact, the Red Line is the newest of the four, so its equipment and facilities are ultra- modern. Thus, as riders sat there on a warm afternoon , waiting to resume their journeys, what do you suppose was causing the delay? A dog. Yes, a dog. An alert observer spotted it trotting along the tracks as it passed Olivais Station, then Cabo Ruivo, before reaching the end of the line at Oriente, where employees caught and removed it from harm's way. No word on how the canine got into the closed system. But a year ago, a lemur escaped from the city's zoo through a hole in a fence and rambled several miles through the Blue Line tunnel before being recaptured.