The subway tunnel connecting Manhattan to several north Brooklyn neighborhoods could either be closed entirely for 18 months or severely curtailed for three years, NBC 4 New York has learned.

The MTA is expected to present the two options for work on the L train's Canarsie Tube, which runs under the East River and connects several lower Manhattan neighborhoods with several trendy enclaves in Brooklyn as well as some of the borough's poorest neighborhoods, at a public hearing on the project Thursday night.

MTA Chairman Tom Prendergast says the project is "the most impactful capital construction project we've ever had to do," and that while work isn't scheduled to begin until 2019, it's crucial that the 90-year-old Sandy-damaged tunnel be repaired soon.

The chances of a derailment, he cautions, increase the longer the tunnel stays in its current state.

"The worst place seems to be with a derailed train is an under-river tunnel. None of this is unsafe right now, but it does require a high level of inspection and maintenance," Prendergast said.

Add to that the incredible surge in ridership on the L -- currently 400,000 people ride the line each day, triple the number since 1990, making it the 10th largest subway line in the U.S., according to the MTA -- and officials say it's about time the crosstown corridor gets relief.

Nine of the 14 East River tunnels were flooded during Sandy. The L tunnel, also known as the Canarsie Tunnel, was ravaged by 7 million gallons worth of saltwater.

The MTA will present two options for work on the L train tunnel:

3-Year Partial Shutdown in the Canarsie Tunnel, One Tube at a Time

IMPACT: 80-percent reduction of service between Bedford Avenue in Brooklyn and Eighth Avenue in Manhattan, resulting in a "very constrained level of service" on one track; service between Bedford and Canarsie in Brooklyn would remain near normal

18-Month Shutdown of Both Tubes in the Canarsie Tunnel

IMPACT: Shuttle buses and ferries would service the stops between Bedford Avenue in Brooklyn and Eighth Avenue in Manhattan; service between Bedford and Canarsie in Brooklyn would remain near normal.

In the latter scenario, the MTA says it will expand service on other lines to accommodate the crowds displaced by the L train shutdown by:

Increasing M train service by 25 percent

Adding two trains per hour on the J line

Adding subway cars to the G trains to make them full-length

Adding three trains per hours on the G line

Adding 10-percent capacity on the A/C train

The following shuttle options would also be in place for the latter scenario:

Williamsburg Bridge shuttle bus from the J/M/Z trains

Ferry slip to the planned new dock on 20th Street in Manhattan

Extra Select Bus Services described by NYC Transit President Ronnie Hakim as "SBS on steroids"

Riders could also take the G train to Court Square and the 7 to Grand Central

In either case, there will be full shutdowns on nights and weekends before 2019 in preparation for the work, the MTA says.

Depending on the scenario of the L tunnel work, there could be "extreme overcrowding" and "long waits," Prendergast says.

When construction gets underway, the First Avenue subway station will get a new entrance at Avenue A, which residents of the Lower East Side have long clamored for.

And by the time the project is complete, the MTA says it will be able to run more trains per hour, thanks to the new substations and more electrical capacity. Currently, L trains travel through the tunnel 40 times per hour during peak times.

The Bedford Avenue and First Avenue stations will also be ADA-accessible, with new stairs and elevators.

The public hearing on the L tunnel work is being held at 6 p.m. at the Marcy Avenue Armory in Brooklyn. Another hearing will follow at the Salvation Army Theatre in Manhattan on May 12 at 5:30 p.m.

The MTA says after the meetings, it will continue to meet with residents, businesses, community boards, merchant groups and civic associations in both Brooklyn and Manhattan.

Brooklyn residents have already held their own community meetings to express their displeasure over a potential shutdown.

Before the L tunnel work begins, the M train renovation must take place.

Hakim said that the city could risk losing hundreds of millions of federal Sandy dollars if they don't get the repair work underway.

Some work on other Sandy-damaged tunnels was done during nights and weekends, while the R line's Montague Tunnel under the East River was closed for more than a year, and the G line's Newton Creek tunnel was closed for two months, both for complete renovations.