One-sixteenth of an inch. That could be the distance between “safe” and “unsafe” for a Metro rail car.

It’s the degree by which Metro technicians measure wheelsets to check for a troublesome and unusual defect that forced about 60 percent of the agency’s rail cars out of service almost one month ago, creating a severe train shortage for the nation’s third-largest transit system that has frustrated tens of thousands of riders with long waits and no indication of when it might end.

On Tuesday, the transit agency held a demonstration of how it hopes to resolve its most challenging crisis in six years. The agency put on a mock inspection similar to the ones Metro hopes will be the key to reinstating most of its 748 suspended cars.

While the malfunction that tilts rail car wheels mere fractions of an inch outward is the root of the setback, Metro officials said finding the right solution is an exacting process that requires precision — and that takes time.

“This is a process that’s methodical,” Metro General Manager Paul J. Wiedefeld said. “It takes a number of checks and balances but that’s what it’s going to be. You’re not going to get on one of these trains until we feel it’s safe.”

The National Transportation Safety Board continues to investigate the cause of the wheel defect, which makes cars more prone to derail. The flaw was exposed after a rail car on a Blue Line train slipped off the tracks Oct. 12, prompting the evacuation of 187 passengers. A federal safety board-led investigation revealed that one of the car’s wheelsets — an axle and two connected wheels — suffered from the defect and learned Metro mechanics had been finding the problem since 2017 within the fleet of the agency’s latest, most advanced and otherwise most dependable model of rail car, the 7000 series.

The Kawasaki Rail-built series, which Metro began phasing into its fleet between 2015 and 2020, popped up twice for wheel defects during Metro’s routine 90-day inspections in 2017. Inspections in subsequent years uncovered similarly small numbers of the defects, which caused transit officials to believe the problem was isolated. But this year, Metro inspections found 18 cases of the issue before the Blue Line train derailment put a spotlight on the growing systemic problem.

Emergency inspections by Metro and the NTSB found about 20 more cases last month, propelling the Washington Metrorail Safety Commission to order Metro to pull all 7000 series cars from service.

Kawasaki and ORX, the company that assembled the wheelsets for the 7000 series, are part of the investigation, and metal from wheels and axles are being tested at a federal lab, NTSB spokeswoman Jennifer Gabris said.

The order from the safety commission, an independent government agency Congress created specifically to monitor Metrorail safety after years of lapses, allows Metro to start using unaffected cars again — if the transit system can show the commission that it can do so without risking passenger safety. Investigators said the defect is progressive and appears only after an unknown length of miles, making it hard to predict. Metro has proposed speeding up inspections of every car from 90 days to every eight days, saying that might be the right formula to catch the first signs of the defect before it poses a danger.

Last week, it began testing that hypothesis on two empty, out-of-service 7000-series trains allowed to run on the system’s tracks, carrying only weighted boxes to simulate passengers. Transit officials said the testing has not ended, and they can’t make an assessment yet.

Wiedefeld said the tests, which could take a couple of weeks, are ongoing. Metro officials have said riders should expect the current limited service to continue at least until the end of the month.

Tuesday’s wheelset inspection demonstration was held at Metro’s Greenbelt rail yard complex in Building B, a giant warehouse where several rail cars, including four from the 7000 series, sit on lifts. Two blue jumpsuit-clad rail car maintenance technicians approached a bare wheelset Metro had placed near the center of the garage on a turntable. Weighing 2,495 pounds, the wheelset was composed of a thick, cylindrical silver axle with two broad, round gleaming silver wheels that looked like discs with rims that jutted upward on their inner face. Two bearings and a gear unit were fixed to the axle.

The maintenance technicians carried a long metal gauge that looked like an oversize tuning fork or trident. With gloved hands, they laid the gauge over the axle and carefully extended it until it touched the inner face of each wheel. Measurements just like a ruler’s adorned the end of the gauge. A technician called out the measurement to the other, who wrote it down.

A measurement was taken two more times on the axle at different spots of the wheels’ circumference.

The span or gauge between wheels corresponds with specifications of the track, said Shushil Ramnaress, Metro deputy chief mechanical officer. The defect pushes some wheels to move outward, which can cause cars to slip. In the case of the derailed Blue Line train last month, investigators said it actually slipped off the track unnoticed twice during the day, getting bumped back on at switches, before its final derailment.

Wheels on 7000 series cars are pressed to the axles with 65 to 95 tons of force so that they should not shift beyond one-sixteenth of an inch and remain 53 5/16 inches from each other, Ramnaress said. Investigators on the NTSB team, railroad-accident experts and longtime mechanics have said they have never seen wheels shift as they have with the 7000 series and have been baffled.

“For all of our fleet, what is critical on legacy [trains], as well as the 7Ks [7000 series cars],” Ramnaress said, “is that the wheelset back-to-back dimension remains in that defined tolerance.”

Each rail car has four axles, resulting in nearly 3,000 gauge inspections required for the entire 7000 series fleet. While conducting the inspections every 90 days did not present too much difficulty, doing them at least 10 times more frequently, as Metro is currently proposing, would be a staffing and logistics challenge. Ramnaress said it takes a team of three people — including a quality-control inspector — six hours to do measurements for 16 cars.

Wiedefeld said Metro is still testing out the entire process, including whether eight days is the right frequency for inspections. With so much still being worked out, he said, he could not predict or provide any estimates on when the entire series would back into service.

“We’re doing that as part of the testing right now,” he said. “We’ll see where the data leads us, and then we’ll determine what’s the right variation.