A sign indicates the closure of the Metro subway station at Johns Hopkins Hospital. (Jerry Jackson/Baltimore Sun)

The sudden, month-long shutdown of the Baltimore subway last week was prompted by a review of track curves that found 17 of 19 evaluated track segments to be in “black condition,” meaning trains could not safely operate on the rails, the Maryland Transit Administration said.

Continued operation of trains in those segments and two others nearing black condition would have posed a derailment risk and could have put the system’s 40,000 daily riders in harm’s way, an inspection report preceding the closure found. MTA officials revealed that information Thursday, amid repeated inquiries for the cause of the closure announced at the end of the past week.

MTA Administrator Kevin Quinn said when he was presented with the report on Feb. 8, he had no option but to shut down the system. Further inspection that weekend found similar conditions underground, he said, and by Feb. 11, the agency announced the entire system would be shuttered for up to four weeks while urgent repairs were being made.

The 19 segments that raised alarm were located in 11 curves throughout the aboveground section of the 15.5-mile system.

“We identified that this was a huge safety issue, and I think we started putting an operations plan together to really think about how we were going to let riders know about this,” Quinn said.

MTA was aware that tracks were deteriorating in the area and had scheduled aboveground rail replacements for summer 2018, at a cost of $20 million to the state. But the January inspection, conducted by an outside firm, revealed the repairs were more urgent than agency administrators had previously thought.

The problem centered on a factor called gauge face angle, where the train’s wheels come into contact with the rails. The inspection report recommended MTA take “immediate action to enact emergency repairs to their tracks.”

As “wear occurs, the gauge face of the rail goes from vertical to sloped,” MTA said. In the 19 sections identified in the inspection, the angle was sloped such that wheels could have skidded off the tracks, based on MTA and Federal Transit Administration standards.

“In our case, it was on curves,” Quinn said. “It was on elevated tracks on curves of the system where trains certainly push against the rail with more force as they’re going around the curves at high speeds.” Quinn said he decided to close the entire system “out of a pure abundance of caution.”

The repair work consists primarily of rail replacements; 33,000 linear feet of new rail will be laid above ground and 6,000 linear feet of rail will be installed in curves within the tunnels, he said. The system is expected to remain closed through March 11, though Quinn said some segments could reopen before then.

The subway’s sudden closure brought widespread attention to the 35-year-old system, which passes through Baltimore’s downtown core, linking Johns Hopkins Hospital on one end with the northwestern suburbs on the other. The aboveground section runs from Mondawmin to Owings Mills.

The one-line, 14-stop subway is not a comprehensive rail network. It carries 40,000 passengers daily, compared to 250,000 who ride the bus and 22,000 who turn to the light-rail line in Baltimore. Gov. Larry Hogan (R) provided $2.2 million in emergency funding to run shuttle buses along the subway route, which would complement existing buses and other transit links around the stations.

The Washington Metro system, in contrast, carries rail passengers on more than 600,000 daily trips. Metro General Manager Paul J. Wiedefeld served as MTA administrator from 2007 through 2009.

But with the sudden shutdown, the Baltimore system became a symbol of problems faced by transit systems throughout the United States, transit experts said.

“It mirrors the problem of the infrastructure of the country,” said Larry White, the former manager of Maryland’s state safety oversight program for the subway, who now works as an independent transportation consultant. “You just have maintenance, you have funding, you have timetables to meet. All those things kind of go hand in hand.”

He said catching up on maintenance was the system’s most urgent problem when he departed the Maryland Department of Transportation in 2016.

“When you have a system this size, of course you have to be up on maintenance,” he said. “If I had to have a railroad system today, that’s where the bang would meet my buck, making sure that maintenance is input . . . . You have issues that are basically systemic . . . where maintenance schedules are not being met.”

“Now is it the scale of like a [Washington Metropolitan Area Transit Authority]? Not really,” he said, “but at what level do you say it’s not significant?”

The Baltimore subway has faced its own unique set of problems in recent years.

More than a year before safety issues forced the recent emergency shutdown, a letter from Rep. Elijah E. Cummings (D-Md.) warned of “grave concerns” about the safety and health of riders and workers in the MTA Metro SubwayLink system.

Cummings’s letter to White, when White was the state safety oversight program manager, highlighted myriad urgent safety concerns with the system.

“The most dangerous hazards include: exposed and decayed wiring, open junction boxes, expired or empty fire extinguishers, uncontrolled water leakage, flooding at and near electrical closets and control panels, failed high voltage conduits, decaying power circuits, fire hazards in the fire control room, and decomposing smoke detectors,” Cummings wrote.

And that was just at one station (Mondawmin, on the northern portion of the underground segment).

In March 2016, fears about corroded electrical cabling forced the abrupt, one-day shutdown of the Washington Metro system. Emergency closures of that sort, and extended, highly disruptive capital improvement projects are becoming more commonplace as urban rail systems age.

Robert Puentes, president of the Eno Center for Transportation, said in an interview this week that the agencies’ propensities for sudden closures speak to a “culture change” transit systems are undergoing:

“ ‘We’re going to do it now, we’re going to do it faster, it’s going to be inconvenient,’ ” he said of the change. “We’ve done a good job in this country of building new stuff, and we continue to do that. What we’ve not done a good job at is upkeep and modernization of the system. These are not designed to last in perpetuity. They have to be maintained and rehabbed, and we haven’t done it.”

David McClure, president of Amalgamated Transit Union Local 1300, which represents the Baltimore subway employees, said workers had raised alarm about the conditions of the tracks. They had observed the conditions in curves and warned management to address them, he said.

“The train will wobble,” he said. “It will lean from side to side — that’s why they go slow over those particular areas.”

He said speed restrictions were put into place in many of the areas of concern, and train movement was reduced from 60 mph to 15 or 20 mph.

“There are some very dangerous conditions here that we are faced with,” he said.

Quinn pushed back against notions that management wasn’t responsive to workers’ concerns or that the sudden closure indicated the subway system was not being properly inspected. He said the system is evaluated using a track geometry vehicle every six to 12 months, and manual inspections occur with more frequency — though he could not immediately say how often.

Many of the areas found to be problematic were first identified in 2015 and were expected to last through summer 2018, when the replacement was scheduled. But for many of the rail segments, which date to the system’s opening 35 years ago, the full rail replacements will come sooner than expected.

“It just so happened that this physical inspection was the one that really revealed that the track simply would not last another six months for us as originally anticipated,” Quinn said. “We were aware of this. We funded [the project] and unfortunately it was a much higher rate of wear than we anticipated.”