Passengers react as smoke fills a Metro train in a tunnel outside the L’Enfant Plaza station Jan. 12. (Saleh Damiger)

On that rain-soaked winter Monday, shortly before the start of the evening commuter rush, Metro’s Yellow Line train No. 302 rolled out of the L’Enfant Plaza station and entered a southbound tunnel toward Virginia. It was Jan. 12, about 3:15 p.m., and disaster soon struck, a fatal calamity that has yet to be fully explained.

The six-car train, with about 250 passengers, stopped in the darkness after encountering heavy smoke. Its lead car was 836 feet into the tunnel, federal safety investigators later said, and the rear of the train was 386 feet beyond the station’s platform.

This week, as the National Transportation Safety Board conducts its first public hearings on the incident, it will be pointed out that as smoke permeated the six cars — sickening at least 86 riders, one of whom died — the train stayed put in the tunnel, never budging from the noxious haze despite the operator’s apparent attempts to back up.

Why train No. 302 remained stationary while passengers gasped for air is one of many questions lingering since Jan. 12. Now, five months into an NTSB investigation that is a long way from being finished, some answers could emerge when the safety board offers the public its first close look at what the inquiry has found so far.

The hearings — set to take place 9 a.m. to 5 p.m. Tuesday and Wednesday at the NTSB’s headquarters in Southwest Washington — will include presentations by safety investigators and discussions about evidence gathered in the tunnel and elsewhere. Voluminous records, including technical reports and interview transcripts, are due to be released by the board.

After being stuck in a smoke-filled train during the Jan. 12 Metro incident at L’Enfant Plaza, Navy officer Jeffrey Todd walked more than 1,100 feet from the train inside the tunnel toward Virginia. Todd exited the tunnel through a vent shaft at Ninth and Water streets SW.

The NTSB inquiry is among several reviews of the beleaguered transit authority by outside agencies and experts since the deadly calamity. The Federal Transit Administration on Wednesday cited major flaws in Metro’s safety-management programs, especially involving the Landover, Md., facility where train traffic is monitored in real time.

Other reports — including one by the Government Accountability Office, requested by members of Congress — are expected to be published in coming weeks.

The Jan. 12 crisis, it seems, resulted from a cascade of breakdowns and failures entailing not just equipment and infrastructure. Metro personnel responsible for the upkeep and operation of the nation’s second-busiest subway also are implicated.

Except for occasional bulletins regarding urgent safety problems discovered during the investigation, the NTSB has been tight-lipped about the incident, saying its final report will be issued early next year. Metro has been circumspect, as well, citing federal rules that bar the transit authority from disclosing details of an ongoing NTSB inquiry.

For two days, however, the silence will be broken, at least to a degree. Here are some of the still-murky issues that could come into clearer focus:

Why did the smoke build up?

Generally speaking, there was “an electrical arcing event,” the NTSB said in a safety bulletin, meaning that electrical current escaped from heavily insulated cables along the track bed and began flowing dangerously around the tunnel.

Metro jumper cables

The arcing, which generated tremendous heat, occurred about 1,100 feet in front of where the lead car of train No. 302 came to a halt. The resulting thermal damage to tunnel infrastructure created a huge amount of smoke, according to investigators.

But how did the electrical current get loose? Although “additional analysis is needed before a cause can be determined,” the safety board said recently, an NTSB document released June 8 contained a possible explanation. It cited Metro’s failure to ensure that all power-cable connections in the subway are protected from contaminants.

The contaminants (water, metal dust and other substances) can seep into a cable connection and form a pathway for electricity to flow out. Whether the hearings will shed more light on the issue of improperly constructed cable connections, along with other potential reasons for the arcing, remains to be seen.

“The smoke in the tunnel,” the NTSB document said, “was generated by thermal damage to about 16 feet of electrical power cables and insulation, portions of 4 fiberglass cable connector covers” and about five feet of fiberglass that was covering the electrified third rail, from which trains draw propulsion power.

The hearings could produce details about the chemical makeup of the smoke from the smoldering insulation. That’s a question of particular interest to passengers who choked on the fumes for more than 30 minutes, some of them clinging to consciousness, as they waited for firefighters to reach the train and begin an evacuation.

A few of the riders left the cars and reached safety on their own. The rest inhaled an array of gases. Officials haven’t specified what the gases were. But an indication might be found in the autopsy report on the deceased passenger, Carol I. Glover, 61, of Alexandria, Va., who suffered acute respiratory failure due to smoke exposure, according to the D.C. medical examiner’s office.

It’s not known, however, whether details of the postmortem report will be included in the information made public this week.

Not long after the incident, Metro announced a plan to replace many subway cables with new, “low smoke/low halogen” cables that are designed to reduce emissions of irritating, even poisonous fumes during an electrical meltdown.

Why did the train enter the tunnel?

In what apparently was the first indication of a problem the near the L’Enfant Plaza station that day, according to the NTSB, a tunnel smoke detector activated at 3:04 p.m. — about 10 minutes before train No. 302 pulled out of the station, headed toward the mass of fumes.

“Recorded data shows that at about 3:06 p.m., an electrical breaker at one end of a section of third rail tripped,” an NTSB safety bulletin later said.

That was another sign of trouble.

Metro’s subway operations are monitored in real time by train controllers in the Rail Operations Control Center, called the ROCC, in Landover. Ideally, experts said, controllers at the ROCC would have reacted to the warnings, especially the smoke-detector alert, by immediately halting train traffic into the tunnel. But they didn’t.

A review of transit authority records by The Washington Post found that months before the smoke incident, Metro knew that train controllers were poorly served by computer software that dates to 2002. The ROCC’s computers generate so many needless alarms, about smoke and other issues, that the warnings often go unheeded.

Although the agency has long planned to buy and install new software, the procurement process, still in the paperwork stage, has moved at a glacial pace.

In its report on Metro’s safety-management systems, the Federal Transit Administration also cited the inadequate software, along with many other problems at the operations center, including understaffing, insufficient training, poor communications among train controllers and unnecessary noise and distractions.

But the FTA did not specifically look at the events of Jan. 12. Part of the NTSB inquiry is focused on how the control center’s shortcomings factored into the L’Enfant Plaza crisis by allowing train No. 302 to proceed into the smoke-filled tunnel. And some of what the safety board has learned could be revealed in the public hearings.

Why didn’t the train back up?

The maneuver seems simple.

On Feb. 11, a month after the L’Enfant Plaza calamity, a similar but less severe incident occurred in Virginia, starting at 1:29 p.m. An Orange Line train “traveling from the Court House station to the Rosslyn station reported smoke in the tunnel as it approached Rosslyn station,” according to an NTSB safety bulletin. The train driver notified the control center.

A Metro train has an operator’s booth at either end. “The train operator was instructed to reverse ends and move the train back to Court House station, which did occur,” the safety board said. “All smoke was cleared” by 2:50 p.m. “and normal train service resumed.”

By contrast, the Jan. 12 incident was chaotic. Investigators said they have gathered all available records of communications between the ROCC and train No. 302 during the crisis. But what those records show has yet to be made public.

With the train stopped in the tunnel, enveloped by fumes that were rapidly filling the six cars, several passengers later said, they heard the operator speaking into his radio, repeatedly asking for permission to return to L’Enfant Plaza. Over the intercom, the operator urged riders to be calm, saying the train would be backing out of the smoke.

It’s unclear whether a power problem kept the train in place, the control center didn’t authorize a return to L’Enfant Plaza or some other complication arose. Passengers said the operator walked from the front of the train toward the rear booth. And they said the train lurched a few times, as if it were about to start rolling.

But it stayed put, as gasping riders, some on the verge of panic, shielded their eyes and mouths with napkins and scarfs. Within 10 minutes — because Metro had failed to halt rail traffic in the vicinity of the L’Enfant Plaza station — backing up ceased to be an option for the operator of train No. 302. By then, the station was occupied by another train.

The “following train,” No. 510, “stopped at the L’Enfant Plaza station at 3:25 p.m,” the NTSB said. “This train stopped about 100 feet short of the south end of the platform, but its cars were entirely within the station.”

Could train No. 510 have been moved backward, to make room in the station? Apparently not. Because its operator was no longer on board.

Due to the fumes, the NTSB said, “Train 510 was evacuated.”

Why did the smoke blanket the train?

Near the spot where the electrical meltdown occurred, a giant ventilation shaft rises six stories from the tunnel to the street. In the shaft are four fans, each five feet in diameter. Depending on how the blades are set to spin, clockwise or counterclockwise, the fans can pull in or push out massive amounts of air per minute.

The shaft, like the source of the smoke, was about 1,100 feet in front of train No. 302’s lead car. Behind the train, in the L’Enfant Plaza station, two large fans installed below the platform level also are capable of moving enormous volumes of air in and out of the tunnel.

In a safety bulletin, the NTSB described a series of missteps in which ROCC controllers botched the remote operation of the fans, exacerbating the plight of the passengers who were stuck on the train. In addition to the mistakes, two of the fans in the shaft experienced mechanical problems, investigators said.

This week’s hearings could fill gaps in the public record regarding the maintenance of the fans, what went wrong with their operation and the effects on the riders.

If coordinated properly, one set of fans would have worked in “supply mode,” spinning clockwise and pushing fresh air into the tunnel, while the other set of fans, running in “exhaust mode,” or counterclockwise, pulled out the smoke.

Instead, the NTSB said, controllers in the operation center activated the fans at cross-purposes, with both sets pulling the noxious fumes. The platform fans pulled the smoke in the direction of the station, meaning the fumes moved toward the train from deep in the tunnel. Simultaneously, the shaft fans pulled the smoke the opposite way.

Because of this tug of war, the fumes stayed in the tunnel, engulfing the train. “There was not a supply of fresh air to aid in moving the smoke,” the NTSB said.

Making matters worse, the safety board said, “the train ventilation system that draws air from the outside into the cars was not shut off by the train operator.”

Under a Metro rule at the time, an operator was not allowed to turn off a train’s air-intake without authorization from the ROCC. Whether the operator of train No. 302 failed to request permission or did not receive it remains unclear. Metro has since changed the regulation, leaving the air-intake decision to the operator.

As smoke enveloped the train, and poured into the cars through the intake system, sickened passengers waited more than a half-hour for help to arrive, some of them sharing water bottles, others kneeling and sitting on the floors, seeking low clean air.

When D.C. firefighters finally arrived, Lt. Stephen Kuhn later said, the fumes were so thick that they could not see the taillights of the train “until we were right up next to it.”