NJ Transit officials had to cancel several trains and suspend service on the Princeton Shuttle during the Monday afternoon commuter rush due to train crew shortages.

An early morning North Jersey Coast Line train was canceled Tuesday because NJ Transit didn't have crews available.

Cancellations on Monday included four rush-hour trains on the Northeast Corridor and North Jersey Coast Line, which NJ Transit blamed on crew shortages in official tweets to commuters.

NEC train #3947, the 4:33pm from PSNY, is CANCELLED due to crew shortage. Customers to train #3165, ... https://t.co/eZoqbG2Z7W — NJ TRANSIT - NEC (@NJTRANSIT_NEC) July 17, 2017

They were in addition to suspension of the Princeton Junction "Dinky" rail service, which NJ Transit blamed on a "manpower shortage." Buses replaced those trains. NJ Transit officials declined to comment about the specifics of the crew shortage.

The cancelled rush-hour trains immediately caught the attention of commuters on social media, who questioned what was happening. NJ Transit settled contracts with its rail unions last year, averting a crippling strike.

.@CommutingLarry hey Larry, any idea what's going on with all the "crew shortage" issues on NJT today? — [REDACTED] (@SvenAsInBen) July 17, 2017

But an NJ Transit official who is not authorized to talk to the media about the subject said the crew issue was due to locomotive engineers who are slow to pick-up their assignments under a new summer schedule change that took effect on July 10. Engineers have 48 hours to accept an assignment, if they fail to respond, it goes to another employee. Under a new schedule, engineers bid to work certain trains and are selected on the basis of seniority, the official said.

However, while saying the agency had adequate staff to provide service, the official did concede the public was being affected by the cancellations. Under the union contract, employees can stay home from work over a crew change and scheduling issue, the official said. Other rail union members are not involved and less than one percent of engineers are involved, the official said.

"The employees are exercising their right under the agreement not to show up," the official said. "They're just not showing up."

A call to a locomotive engineer's union official and an email were not answered, however it was after business hours.

Larry Higgs may be reached at lhiggs@njadvancemedia.com. Follow him on Twitter @commutinglarry. Find NJ.com on Facebook.