PHILADELPHIA — An ocean liner that was the biggest and fastest ever built in the United States, and a symbol of national prestige until it was left to rot on the Philadelphia waterfront, may soon sail again. Norwegian Cruise Lines said Monday it has purchased the SS United States and intends to refurbish and return it to service, a half-century after the massive steamship set a speed record for a trans-Atlantic crossing. The announcement amazed preservation groups, who had been fighting what looked like a losing battle to rescue the ship from being scrapped. "I'm stunned," said Susan Gibbs, the...