Subway cars undergoing inspection after mysterious 4 train decoupling in the Bronx

New York City Transit is inspecting over 1,000 subway cars on the Nos. 2, 4 and 5 lines after an unexpected decoupling left more than 200 passengers stranded in a Bronx Subway tunnel Sunday.

“We have a very rigorous inspection protocol for our trains, and in addition to our normal day-to-day inspection of trains we are also going to inspect all of the R142 car-class to ensure that there’s no other issues remaining with the cars,” NYCT president Demetrius Crichlow told reporters Monday.

The first three cars of a southbound No. 4 train decoupled from the rear seven cars just after 5:20 p.m. Sunday night while the train was between stations near 149th St. – Grand Concourse in the Bronx.

The decoupling triggered the emergency brakes on the train, bringing both sections of the subway to a halt in the tunnel, stranding those onboard.

Rescue trains were dispatched to bring passengers safely to nearby stations. A spokesman for the FDNY told the Daily News that 60 people were evacuated from the first three cars of the train, and 178 were taken off of the rear seven cars.No injuries were reported.

Crichlow said Monday that the cause of the incident was still under investigation. A source with knowledge of the incident told The News Monday that an initial assessment blamed “mechanical failure” for the train’s separation.

Like most cars in the New York City subway system, the R142 comes assembled as a set, with five cars linked together. The cars are secured through a complex mechanical, electrical and pneumatic linkage. That linkage is very rarely separated during normal maintenance, sources familiar with subway upkeep told The News.

But a source with knowledge of the maintenance history of the particular cars involved in Sunday’s incident told The News that the set that spontaneously decoupled was at the 207th St. Yard in Manhattan in June of this year for a major overhaul.

That work, the source said, does involve disconnecting — and ostensibly then reconnecting — the five-car set.

It was unclear Monday whether the MTA was investigating whether the cars were put back together properly in June. A spokesperson for the transit agency did not immediately respond to questions about the cars’ maintenance history.

The train-linkage system on the R142 is similar to that found in the R142A cars of the No. 4 train and the R188 cars of the No. 7 train. The MTA did not immediately respond Monday as to whether those car classes were being inspected as well.