The R train is back in full force Monday as of 6 a.m. and will start carrying riders between Brooklyn and Manhattan now that work on the underwater tunnel damaged during Superstorm Sandy is complete before schedule.

Gov. Andrew Cuomo announced the return of full R train service Sunday at the Whitehall Street station, the last stop on the line that ran from Queens through Manhattan since work began on the 4,000-foot Montague tunnel in August 2013.

The R train's return is weeks earlier than anticipated at about $60 million below cost, totaling $250 million, according to transit officials. During Sandy, 27,000 gallons of saltwater poured into the tunnel, corroding equipment and rails that led to service delays.

"It's laudatory that they got it done and they got it done as quickly as they did," Cuomo said. "It also reminds us how challenging it's going to be to rebuild this system."

Repair work on the transit system's damaged underwater tunnels has been a burden to riders. The closure affected 65,000 daily commuters that took the R train to and from Manhattan.

As the Montague tunnel underwent a full 13-month shutdown, the Greenpoint tunnel on the G line was shut down for 12 weekends last year and for five straight weeks in August. Nine underwater tunnels were flooded during Sandy and more repairs in those tubes are expected, including work on the Cranberry tube on the A and C lines, and the Rutgers tube on the F line, though transit officials have said a full shutdown of service can be avoided.

The Straphangers Campaign in a statement lauded the MTA for putting an early end to the burden on R train riders.

"Tens of thousands of riders were forced to change how they commuted to work," the Straphangers Campaign said. "For many, this meant losing a 'one-seat ride' or having to navigate additional stairs and badly congested platforms."

Cuomo joined MTA chief Tom Prendergast and top engineers inside a special train car from Brooklyn into Manhattan that lit up the tunnel for a look at the renovations, according to a pool report.

Crews replaced 11,000 feet of track, 75,000 feet of power cable and 200,000 feet of communication cable. They also girded the system against future storms by moving a signal room at Whitehall Street to higher ground, sealing a circuit breaker room that powers tracks with a submarine-quality door, using water-resistant power cables and installing stronger pumping equipment.

The MTA still has work to do. About 500 locations in lower Manhattan where water can flood into the transit system still need to be sealed off.

"We're better prepared in terms of knowing what to do and how to protect the system," Prendergast said, "but more work has yet to be done."