The BLET union said picket lines will begin at Penn Station in New York City, the Atlantic City Rail Terminal and NJ Transit headquarters in Newark at 4 a.m. ET. The union represents 51,000 engineers and other train service workers across the U.S.

The union that represents the engineers who drive New Jersey Transit’s commuter trains has threatened a walkout as soon as Friday if a contract deal is not reached.

NJ Transit engineers are preparing to walk off the job on Friday, which could affect 350,000 commuters in New Jersey and New York City.