Although Hurricane Sandy had been downgraded to a tropical storm by the time it hit New York City in October 2012, the storm surge it brought devastated the coast. Among the worst hit were the city’s swamped subways: Water devastated power stations and switching equipment. The Metropolitan Transportation Authority suffered billions of dollars in losses.

Last April, when it came time to renew the MTA’s insurance policies, many longtime insurers were no longer willing to cover the agency’s risks. Others doubled or tripled the premiums, or reduced coverage.