How bad is the post-Sandy damage to the tunnel that carries the L train under the East River?

"You have to demolish this tunnel," MTA president Tom Prendergast told state lawmakers during a joint legislative budget hearing in Albany Thursday.

That means the tunnel needs to be taken down to the studs — or in this case the steel structure lining the tunnels. But because of the complexity of the work, Prendergast said the Canarsie Tube will need to be completely closed while it is being overhauled.

"Demolishing the concrete creates silica dust," he explained, "and from an environmental standpoint, for the customers and the employees and the workers, you have to protect them."

The tube, which was constructed in the 1920s, contains two tunnels (one eastbound, one westbound). It was one of eight subway tunnels to be flooded during Sandy, when it took on 7 million gallons of salty water. After the storm, it was closed for 11 days.

But unlike the other subway tunnels damaged during Sandy, the Canarsie Tube presents unique challenges. "It's the number of people impacted," Prendergast said, "and there are much fewer alternatives for them to utilize. That's what makes it different."

The MTA doesn't expect work to begin before late 2018 or early 2019. But Prendergast said the agency will start letting out contracts this year, because it's using federal money and a new Congress is coming in.

"If we don't commit those dollars this year," he said, "there's a likelihood that they could be snatched back up and used for other purposes."

The hearing answered numerous questions raised by angry Brooklynites during a public meeting last month. An MTA official present did not know when the repair work would begin and whether the closure could be limited to weekends — or even when the MTA would be able to answer those questions.

He said the agency was working on alternative transportation plans for L train riders, as well as ways to compress the work into as short a time as possible. But the duration of the Canarsie work will take "a number of months."

On average, 250,000 people a day take the L train under the East River, with a total of 400,000 people using the entire line.

You can see an MTA video of work on the Canarsie tube in the aftermath of Sandy, below.