Sound Transit will perform a “root cause analysis” to figure out why all four high-speed elevators at Beacon Hill Station shut down Monday afternoon, forcing trains to skip the South Seattle stop all evening and into Tuesday morning.

Technical staff don’t know yet what caused the apparent software failure, said agency spokesman John Gallagher.

When one elevator, known as the “master,” shuts down, the other three can quickly follow, he said. There was no obvious event such as a power outage or surge, or a vehicle hitting power equipment, that triggered the shutdowns.

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Crews worked overnight to get two of the elevators operating by 8 a.m. Tuesday, so the station could reopen for the later part of the morning commute. A third restarted in time for the afternoon commute, he said.

Passenger boarding platforms are 160 feet deep, so train customers there depend entirely on the four elevators. Stairs exist for emergencies and maintenance workers only.

The outage, which began at 4:30 p.m. Monday, followed a shorter Monday-morning outage that the agency attributed to a false fire alarm.

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The afternoon shutdown was “a much more complicated problem,” Gallagher said. However, teams believe they can diagnose or fix any remaining flaws overnight, while trains aren’t operating, he said.

The elevators were built and installed by the KONE company. Individual elevators failed or were stopped for repairs 324 times at Beacon Hill Station during 2017-18, out of 3,148 overall at seven stations, according to logs released to The Seattle Times under a public records request.

Serious disruptions are rare, although elevator failures did close Beacon Hill Station one day in January 2016.

The elevators are currently maintained by Schindler, which last month won a five-year, $23 million contract to maintain 139 Sound Transit conveyances throughout the region, a move the transit board hopes will improve reliability.

Beacon Hill Station serves just over 3,000 daily passengers, slightly more than Rainier Valley stops a short distance south. The entire line from Husky Stadium to Angle Lake serves around 80,000 riders.

During Monday’s closures, Sound Transit operated buses to shuttle people between stations. As usual, passengers tweeted that communications were garbled, and a few said Sound Transit should open nonemergency stairs, as the agency did this year for the shallower University of Washington and Capitol Hill Stations.

Meanwhile, frequent escalator and elevator failures continue to frustrate passengers, especially at downtown stations. Sound Transit says it will spend at least $52 million for replacements, after it takes over the downtown tunnel from King County Metro in 2020. Schindler will also take over maintenance downtown in 2020.

That’s too late for Connect 2020 in January, when construction at the International District/Chinatown Station to link the existing line to tracks to Bellevue will force 10 weeks of lower train frequency, and more passenger transfers downtown.