The R train is running between Manhattan and Brooklyn for the first time in more than a year.
Trains are once again making their way through the Montague Tunnel, which was shut down for 13 months to repair damage from Hurricane Sandy.
"I'm like really excited. I was telling my son, like, 'Oh my God, it's back, it's back,'" said one excited rider during Monday's morning rush.
"It's been pretty difficult. I'd need to take three trains to go to work. Now I just take two," said another rider.
Governor Andrew Cuomo took a ride through the tunnel Sunday to get a closer look at the renovations, which are meant to strengthen the tunnel against future storms.
The governor says the $250 million project came in under budget and ahead of schedule.
"Besides the shell of the tunnel, the whole internals had to be reconstructed. All the cabling, all the wiring. And it's laudatory that they got it done and they got it done as quickly as they did," Cuomo said.
Thousands of feet of rusted rails were replaced in the tunnel.
Workers also installed new signals and lights, along with new pumps that can remove 1,900 gallons of water per minute.
"We took out everything that was in there: The cables, the wires, the rails, the third rail, everything down to the track bed, and replaced it all. And remember, we had to do all of this while bringing workers and equipment in through the tunnel and bringing all the debris back out," said MTA Spokesman Adam Lisberg.
An estimated 27 million gallons of water flooded the tunnel during Sandy.
The R's full-line restoration comes less than two weeks after the MTA wrapped up a five-week shutdown of the G train tube that runs between Brooklyn and Queens, also for Sandy-related repairs.