Tens of thousands of commuters are bracing for a difficult trip around southwest Connecticut and to New York City on Monday as workers repair the Metro-North commuter rail line crippled by a derailment and crash. Crews will spend days rebuilding 2,000 feet of track, overhead wires, and signals following the collision between two trains Friday evening that injured 72 people, Metro-North president Howard Permut said Sunday. Nine people remained hospitalized. Several days of around-the-clock work will be required, including inspections and testing of the newly rebuilt system, Permut said. The damaged rail cars were removed from the tracks on Sunday, the first step toward making the repairs. Service disruptions on the New Haven line between South Norwalk and New Haven are expected to continue ‘‘well into the coming week,’’ Permut said. NTSB investigators arrived Saturday and are expected to be on site for seven to 10 days.