National Grid will be replacing natural gas lines to service a new development just over the town line in Belmont, but the project will impact users of two main roads in Watertown. A representative from the utility appeared before the Town Council earlier this month to discuss the portion of the project in Watertown, which will go down Lexington Street. Then the project continues to Belmont Street. In Watertown, the work will replace 350 feet of 12 inch gas line from Lexington Street at Warren Street to the island at Lexington and Belmont streets. A representative for National Grid at the meeting said the new gas line is needed to allow capacity for a new development in Belmont.