A total ban on smoking was imposed by two commuter rail lines that carry 200,000 passengers a day into New York City, but only one major incident was reported--a male passenger punched a conductor in the face when told to put out his cigarette. However, traffic was light on Presidents Day, and the real test will occur today, when passenger volume returns to normal. The ban was imposed by the Metropolitan Transportation Authority on the Long Island Rail Road and the Metro North line under a threat of the loss of $539 million in federal transit funds. Previously, some cars had been set aside for smokers, and a group of smokers has filed suit to rescind the ban; a hearing has been set for Feb. 25. The smoker who slugged the conductor was arrested.