Riders wait on a crowded platform at Metro Center on Monday after service was disrupted by an electrical fire in a tunnel just west of McPherson Square. (Luz Lazo/The Washington Post))

An electrical fire in a Metro tunnel early Monday, which caused huge delays on three subway lines, involved the same type of track-based power cables that burned during last year’s fatal Yellow Line smoke incident in another tunnel, the transit agency said.

The fire, which broke out about 4:30 a.m. just west of the McPherson Square station, fouled the morning and evening commutes for thousands of riders on the Orange, Silver and Blue lines. While repair crews worked in the tunnel, the frequency of trains on the three lines was greatly reduced, and some stations were bypassed.

The agency planned to suspend service between the Foggy Bottom and Federal Triangle stations beginning at 9 p.m. to complete repairs by Tuesday.

Riders tweeted photos of dangerously crowded platforms, and there were reports of passengers becoming ill in the long waits and close quarters.

Tweeted one disgruntled rider during the evening commute: “45 minutes since last [Silver/Orange/Blue train] at L’enfant. No excuse for that. None. Zero. Zip. Nada.”

Said another, upon learning that Monday’s problems were similar to those of last year’s Yellow Line incident: “Today convinced me that I will never, ever bring my child onto a metro train. Not a risk I’m willing to take with a kid.”

Monday’s day-long mess was the latest in a spate of chronic service disruptions plaguing Metro and aggravating long-suffering commuters, especially over the past year.

“I know that it was a tough commute,” General Manager Paul J. Wiedefeld said in a statement Monday afternoon, announcing that the service disruption would extend late into the evening. “I know that many of you experienced delays of 30 to 60 minutes,” he said. “I apologize for the delay, inconvenience and crowding.”

Metro spokesman Dan Stessel said the agency does not plan to offer fare refunds, as it has in the past after severe service problems.

“As always, such requests are handled on a case by case basis,” he said in an email. Monday’s fire occurred before dawn, so “fortunately there was plenty of advance notice of the disruption before most customers got to the system.”

Wiedefeld said in an interview that the fire was caused by an electrical malfunction involving power lines called “jumper cables.”

In that sense, the incident seemed to echo the deadly calamity on Jan. 12, 2015, near Metro’s L’Enfant Plaza station. An electrical malfunction on tracks near the station that day filled a tunnel with smoke, engulfing a stalled Yellow Line train in fumes. Scores of passengers were sickened and one died of respiratory failure.

In many places in the subway, for various reasons, there are gaps in the electrified third rails. Jumper cables bridge the gaps almost like extension cords, allowing current to continue flowing along the third rails, from which trains draw power.

The National Transportation Safety Board is investigating the L’Enfant incident and plans to issue its final report next month. Preliminary documents and photos made public by the NTSB show that the electrical malfunction in the L”Enfant tunnel was centered near a 20-foot gap in a third rail that was bridged by jumper cables, which were destroyed.

Metro said shortly after the L’Enfant incident that it was accelerating a long-term plan to replace many older jumper cables with newer ones.

Monday’s fire also occurred in a third-rail gap that was bridged by jumper cables, Wiedefeld said. “Basically, the older cables, where they connect to the third rail, something happened — a short circuit; I don’t know what the official term is — and it caused the condition of the flare-up, which caused the fire in the actual cables.”

The “short circuit,” as he called it, could trigger electrical “arcing,” which was the cause of the L’Enfant fire, the NTSB has said.

A preliminary safety board report cited “severe electrical arcing damage” to the jumper cables at the L’Enfant site. Arcing suggests that the fire there resulted from electricity escaping from one or more of the cables. This could occur if the cable insulation was damaged, exposing the current to moisture or other contaminants, which could provide a path for the electricity to flow out of its containment.

That could generate tremendous heat, causing smoke-producing fire or melting.

“We don’t want to speculate on what the actual cause was at this point,” Stessel said of Monday’s fire. He also said he had no information on whether the jumper cables near McPherson had recently been replaced.

Many subway power cables, including jumper cables, are attached to other power lines by connector assemblies called “boots.” In June, the NTSB said “a number” of boots through­out the rail system lacked the proper type of “sealing sleeves,” which are designed to keep contaminants away from the electrical current.

These “improperly constructed power-cable connector assemblies” are a significant problem, the NTSB warned. When contaminants create a path for electricity to escape, the safety board said, the result can be “fire and smoke in tunnels.”

After that NTSB warning, Metro acknowledged that about 80 percent of the approximately 6,400 power-cable connector assemblies in the subway lacked adequate sealing sleeves. Rectifying the situation will require months of work, Metro said.

As for Monday’s fire, Wiedefeld said: “We don’t know if it was caused by the boots. We don’t know what it was yet.” Asked whether the boots near McPherson were fitted with proper sealing sleeves, Stessel replied in an email: “Subject to investigation.”

To ease daytime rail congestion Monday, Metro said, Silver Line trains ran only in Virginia, between the Wiehle-Reston East and Ballston stations. That cutback in Silver Line service was to continue at least until Tuesday morning.

Meanwhile, Orange and Blue line trains in both directions were sharing one track between the Foggy Bottom and the Federal Triangle stations throughout the day and night, while repair crews occupied the other track, which was closed.

“I get on near the start of the Orange Line, Dunn Loring, and it was standing room only,” said Paula Hill, 57, referring to the crowding caused by the reduced frequency of trains during the morning rush.

“I stood all the way to Virginia Square before I got a seat,” said Hill, who works at a law firm in the District. “They took us to Foggy Bottom. And we sat and sat and sat and sat at Foggy Bottom. And then they offloaded the whole train.”

She said she crowded onto another train and finally made it to McPherson Square, more than an hour after beginning what is normally a half-hour commute.

“I’m very tired, and I’m not even at work yet.”

Amid the confusion of single-tracking and station-skipping, Nana Amoakohene, an information-technology consultant who lives near McPherson, was trying to get to his office in Rosslyn. “Someone just barked instructions at me, and that’s all I got,” said Amoakohene, 36, after an encounter with a McPherson station manager. He was leaving the station moments after entering it.

“I don’t understand what they’re trying to tell me,” he said. “I don’t have a car, so this is my only form of transportation, the way I get to work.”

“I’m taking a taxi instead.”

Chuck Moran, 23, who lives near Logan Circle in the District and works for a technology company in Tysons Corner, faced the same confusion at McPherson. Moran was headed out of the station moments after going in. “I’m just going to go try to work at a coffee shop until they get all this sorted out,” he said, calling the situation “unacceptable.”

Metro plans to establish a grace period for commuters in situations like Monday’s, allowing them to leave a station within 15 minutes of entering without having to pay the minimum fare. But that won’t start until July.

“It doesn’t feel like anyone cares about the riders,” Moran said. “I just walked in and out of these gates and was charged $2.15. And I asked the station manager, and all she said was, ‘Yup, that’s what happens,’ and then just walked away from me.”

At Metro Center, customers on crowded platforms struggled to figure out which trains to take and where to catch them. “Look at this,” said Helena Djordvedic, 42, who was trying to reach the Federal Center SW station. “You don’t know which direction it’s going, you just have to listen to their shouting, and we’re already 15 minutes late.”

Tourists had trouble navigating the maze of service disruptions. Brian Delle Donne, 58, of Morristown, N.J., was trying to visit the Library of Congress. At Foggy Bottom, he said, he boarded a train that stayed in the station for 10 minutes and was then offloaded.

“It has been horrendous,” he said.

Luz Lazo, Faiz Siddiqui and Perry Stein contributed to this report.