NJ Transit and Amtrack are warning their rail customers to gird their loins for a potential full month of misery as a yearslong infrastructure project reaches a critical juncture — while promising the pain will be worth it in the end.

“In just a few short weeks, we will reward the patience of Amtrak and NJ TRANSIT customers by helping eliminate a cause of long delays and unreliable commutes,” Amtrak President Roger Harris said in a statement.

Amtrak on Sunday began the laborious process of transferring rail traffic away from the 115-year-old Portal Bridge spanning the Hackensack River in New Jersey to the newly constructed Portal North Bridge.

The train route is notorious for creating downstream effects affecting the entire Northeast Corridor because NJ Transit and Amtrak share a track north of Trenton. Delays at the junction frequently cause train traffic into and out of New York City to grind to a halt.

NJ Transit train riders will now have to do without direct weekday service into Penn Station on its Morristown, Gladstone Branch and Montclair-Boonton Lines — with all Midtown Direct service to be diverted to Hoboken — for the next few weeks.

Straphangers’ tickets will be cross-honored on the PATH train between Hoboken and 33rd Street, NJ Transit’s 126 bus and the NY Waterway ferry.

Transit officials are warning riders to expect delays because of the changes, particularly in Hoboken, which is expected to be badly jammed during peak hours and with the NJ Transit cross-honoring program.

Amtrak riders will meanwhile see a reduction in service for the month as 280 trains have been adjusted in light of the project’s last big push. The train service’s Acela, Northeast Regional and Keystone routes are among those affected.

The bridge work will be performed on both sides of the span simultaneously and will see 2.5 miles of new railroad connected to existing infrastructure. Two new interlockings will be built, and more than 4,500 feet of new track will be laid.

Amtrak says the job will require about 40,000 labor hours, to be done by 70 to 90 workers per day over the next month.

The work is expected to wrap up by March 14, when regular schedules will hopefully be resumed.

NJ Transit riders can check if their routes are affected here, and Amtrak customers can get help by calling 1-800-USA-RAIL or through Amtrak.com.

The current crossing has long been beset by delays caused by mechanisms locking up, particularly when contending with boat traffic — with Gothamist reporting that sometimes workers have even had to resort to banging the locking mechanism with a sledgehammer to get it working again.

Former Gov. Phil Murphy said during the project’s 2022 groundbreaking that the existing bridge gets stuck one out of seven times it opens to allow maritime traffic to traverse the waterway.

Fortunately, the new Portal Bridge North won’t have to open to accommodate traffic along the river. However completing the project means trains will be operating on a single track between Newark and Secaucus for the next month, limiting service into Penn Station on weekdays, the outlet said.