MTA seeks to crack case of missing F train bolts in Coney Island subway derailment

Four bolts — and their mysterious disappearance — are at the center of the MTA’s investigation into the Jan. 10 derailment of an F train in passenger service in Brooklyn, the Daily News has learned.

MTA investigators are trying to track down the high-strength Grade 8 bolts, which were noticed missing from the undercarriage of the derailed car when crews arrived on the scene on Coney Island, transit sources told The News.

The bolts are supposed to affix to a train’s undercarriage a piece called the “radius arm,” which helps keeps in place subway cars’ massive wheels and axles.

Properly installed radius arms keep wheels correctly aligned as trains move, subway car experts explained.

“If that [radius arm] falls off or breaks, it can derail a train,” said a transit source with knowledge of subway car maintenance.

The fourth car of the northbound F train jumped the elevated track at midday on Jan. 10 as the train approached the Neptune Ave. station.

No one was injured in the incident.

Photos from the scene show one of the fourth car’s trucks — the wheel, motor and brake packages that make up the undercarriages of subway cars — more than a foot off the rails.

Those photos also show one of the subway car’s eight radius arms, boltless and disconnected — an assessment confirmed by multiple transit sources.

Investigators have yet to determine the cause of the derailment, and have not ruled out track issues or other factors. But an MTA spokesperson confirmed Friday that the agency believes the bolts may have contributed to the incident.

It is not yet clear whether the bolts broke before they disappeared, or if they were not properly installed in the first place. Sources told The News that vandalism was not being considered as a factor.

When properly affixed to the train, the radius arm bolts are secured against accidental loosening with a metal cotter pin, according to a diagram of the truck reviewed by The News.

While bolts can loosen over time from the vibrations and mechanical shocks of everyday train service, critical fasteners on trains are checked at regular inspections, said experts on subway maintenance.

The trucks on the car that derailed, a Kawasaki R160, were last inspected on Nov. 21, and had traveled roughly 9,000 miles before the derailment, according to sources.

That’s within the MTA’s maximum allowed inspection interval of every 78 days, or 12,000 miles.

An MTA spokesperson told The News that out of an abundance of caution, some 50,000 radius arm bolts have been checked across the subway fleet since the derailment. The bolts have been checked on R160s and cars of similar design.

A typical ten-car train of R160 cars should have 320 such bolts.

The MTA said no “systemic issues” were found during the inspections, and officials believe riders are not currently at risk.

MTA officials have previously said an issue with the track along that section of the Culver Line — which carries the F train over southern Brooklyn — was the likely cause for the derailment.

Investigators are now looking into whether an issue with the track could have exacerbated the damage to the train’s undercarriage, sources said.

The section of track where the train derailed had passed a walking inspection the day before, and was inspected by the agency’s track geometry car in November.

Days before the Coney Island derailment, on Jan. 4, two slow-moving trains collided on tracks near the W. 96th St. and Broadway subway station in Manhattan’s Upper West Side, injuring 24 people, none seriously. That incident is under investigation by the National Transportation Safety Board.