NJ Transit commuters have been stuck standing in the aisles of crowded rail cars in part because of the challenge finding the parts needed to repair the trains, officials say.

Commuters have complained or several months on social media about overcrowding, especially when trains are missing a car or arriving with single-level cars instead of with two levels.

Unhappy crowded Monday on Morris Essex @NJTRANSIT people standing everywhere on a very short train. Why? @ABC7NY pic.twitter.com/e7jZrW5ZVI — MichelleCharlesworth (@mcharlesworth7) December 4, 2017

NJ Transit officials says it's taking longer to get parts, especially for older trains. NJ Transit was ranked second-worst in the country for train breakdowns by a federal agency in 2016.

"It is taking us longer to get parts," Robert Lavell, NJ Transit rail operations director said during a legislative hearing on Monday. "We have older equipment that is failing more frequently and we're in the process of replacing that equipment."

Despite approvals to buy new locomotives to replace 50-year old diesels and plans to replace 40-year-old rail cars, new equipment is still years away from being built and delivered, meaning that commuters will continue to ride older, trouble-prone trains.

"(Those older) parts are difficult to procure because vendors no longer carry the parts," Lavell said. "Some equipment consumes parts faster than the vendors can bring them to the shelves."

The result is more commuters complaints about standing in packed rush hour trains.

@CommutingLarry @NJTRANSIT #1614 standing room only. Another dangerous situation. Don't they ever learn? People standing in the vestibule again. pic.twitter.com/ZKwUzF2GMA — Neecy (@Dvlwoman30) January 9, 2018

Frustrated commuters post photos of crowded trains on social media. Others complain that trains are too full to get on when they arrive at their station because a car is missing. Many angrily mention the hundreds of dollars they pay for a monthly ticket.

About 230 rail cars and locomotives are out of service at the Meadowlands Maintenance Complex, said Assemblyman John McKeon, D-Essex, who's in charg