Just as the post-pandemic return to offices in New York City has achieved sustained momentum, tens of thousands of commuters from New Jersey are being warned that they may need to work from home if train engineers go on strike later this month.

Amid strained negotiations with the union that represents its train drivers, New Jersey Transit began telling its customers on Wednesday to prepare for a shutdown of the statewide rail service in mid-May. It would be the first such strike in New Jersey in more than 40 years.

The agency laid out a contingency plan that involves substituting buses for some of the trains that carry 70,000 commuters into Manhattan on a typical weekday. But Kris Kolluri, the new chief executive of New Jersey Transit, said that supplemental bus service, from four “park and ride” lots around the state, could carry only about 20 percent of the daily train riders.

“Buses can never satisfy the demand we have on trains,” Mr. Kolluri said, adding that each bus holds about 100 passengers, compared with 1,000 on an average train. “Those folks who have the ability to work from home during the strike should plan to do so.”

Mr. Kolluri, who took over the agency in January, has been trying for weeks to work out a new contract with the Brotherhood of Locomotive Engineers and Trainmen, which represents about 460 New Jersey Transit engineers. Last month, the two sides announced that they had reached a deal, but in mid-April the rank-and-file engineers overwhelmingly rejected the terms.

That rejection started a 30-day countdown to a May 16 strike deadline. Beginning on that date, a Friday, the agency advised its customers to work from home if they can and to limit their use of the rail system to “essential purposes only.”