NJ Transit riders may encounter another summer of train delays and cancellations.

The eight locomotive engineers who will graduate from training in May will not be enough to fix the shortage that has plagued the agency for the past couple of years, Gov. Phil Murphy said Tuesday after a tour of NJ Transit's Emergency Operations Center in Maplewood.

A certain number of NJ Transit engineers will be pulled off their regular runs to test positive train control, he said. As in past summers, more engineers will take time off, further straining the available pool.

The agency won’t reach “critical mass” on balancing out its engineer roster until 25 to 30 trainees graduate in October, Executive Director Kevin Corbett said.

The engineer shortage, plus the impact of retirements and those who left the agency to work for other railroads, contributed to a near-meltdown of rail service last summer.

So did the agency's scramble to finish installing positive train control, a required safety system, by the end of 2018. But NJ Transit isn't done with the work to make the system fully operational, and the ongoing project contributes to the ongoing engineer shortage.

"Engineering class size is more of a risk than positive train control," Murphy said Tuesday.

NJ Transit has recruited more than 100 engineer candidates, who must complete a training class that takes as long as 20 months.

The agency has been operating a reduced train schedule since last June, and the persistence of the engineer shortage makes it less likely it will be able to restore its full timetable this summer.

NJ Transit is bringing back service on two popular routes, the Atlantic City Line and the Princeton Branch, on May 24. Both have been shut down entirely since last fall, with buses substituting for trains.

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But the agency has announced no date for bringing back other service it suspended, including one-seat rides to New York Penn Station from the Raritan Valley Line and weekend service to Gladstone.

Numerous train schedules on other lines have been curtailed as well.

NJ Transit and Metro-North attempted to come to an agreement to allow about a dozen engineers that Metro-North had hired away from NJ Transit to help ease the shortage on the Port Jervis and Pascack Valley lines, which NJ Transit operates for Metro-North.

However, Corbett said Tuesday, they were not able to make the arrangement work.

Last year, NJ Transit eliminated two of Metro-North’s express trains on the Pascack Valley Line, while the Main and Bergen County lines, which serve Suffern, New York, lost four trains.

Thomas Zambito of lohud and The Journal News contributed to this article.

Email: tate@northjersey.com