NEW YORK — If you laid the New York City subway system in a line, it would stretch from New York to Detroit.
Now imagine inspecting every inch of that track.
That’s the job ahead for Metropolitan Transit Administration officials, who must examine 600 miles of track and its electrical systems before they can fully reopen the largest US transit system, which took a direct hit from Hurricane Sandy.
Seven subway tunnels under New York’s East River flooded, MTA officials said. Pumping them out could take days, and a 2011 state study said it could take three weeks after hurricane-driven flooding to get back to 90 percent of normal operations. That study forecast damages of $50 billion to $55 billion to transportation infrastructure, including the subways.
‘‘No subway system is designed for a flood of this magnitude,’’ said Nasri Munfah, chairman of tunnel services at HNTB, an infrastructure construction, design, and consulting firm based in Kansas City, Mo. ‘‘I don’t think it’s going to be a matter of a day or two. It’s a big job.’’
Chairman Joe Lhota of the MTA, which carries an average of 8.7 million riders on weekdays, called the storm the worst disaster in the subway’s 108-year history. Sandy, which made landfall in New Jersey and swept north to New York, was the largest tropical storm ever measured in the Atlantic.
A subway system has one electrical system that runs equipment, pumps, lights, and communications; another that runs switches and signals; and a third that powers the electrified third rail, Munfah said. All can be ruined by salt.
‘‘It’s like dropping your computer into a bucket of salt water,’’ he said.
MTA officials said Tuesday the extent of electrical damage cannot be assessed until the water is drained.
Thousands of connections in signal systems will need to be cleaned and tested before trains can run again, said Mortimer Downey, a former MTA executive director and current board member of the Washington Metropolitan Area Transit Authority.
‘‘It’s an enormous amount of wiring and an enormous amount of connections that go to what’s called relay rooms,’’ Downey said. ‘‘They’ve got to turn the system on, and if it seems to be working I think they’ve got to go to every component and check it and get rid of all the salt. What you don’t want is a short circuit that causes the system to fail.’’
Such a failure caused the deadliest crash in the history of Washington’s Metro system in 2009, he said.
Finding enough replacement parts will be another challenge that could delay repairs, said Kathy Waters, vice president for member services at the American Public Transportation Association.
‘‘The New York system, although there are some components that have been upgraded over the years, has a lot of antique components where the vendor has been out of business for 50 years,’’ Waters said in an interview.
‘‘To some extent, they will have inventory,’’ she said. ‘‘But depending on the extent of damage, you don’t keep things on the shelf to have a replacement for every piece should it all go down at once.’’
A 2011 study funded by New York state concluded it would take 21 days to restore the subway system to 90 percent of its full operations after flooding caused by a severe hurricane.
It estimated it would take a week to pump water out of tunnels, assuming officials could get 100 portable pumps. The subway had only three mobile train pumps for the entire system, the report said.