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A computer module described as being “about the size of a pizza box” was the culprit in a pair of Metrorail stoppages last weekend, the Washington Metropolitan Area Transit Authority announced today.

The faulty module was part of a information-management device that displays trains’ locations on a dynamic map at Metrorail’s command center. Twice last weekend, that module failed, knocking out the map and forcing all operating trains to hold at stations for as long as half an hour.

The first disruption occurred about 2:10 p.m. Saturday. Trains between stations were guided to the nearest stop via radio signals and traffic lights, but remained stopped until about 2:50 p.m. when the map system was rebooted. But the scene repeated itself between 12:30 and 1 a.m. Sunday morning.

Metro’s technicians are still conducting diagnostic tests to determine why the module failed, a spokeswoman for the transit agency said. But the part has been removed and replaced, and trains were running normally this morning, Metro said.

“While we have confidence that the immediate issue has been resolved, our work must and will continue to make this system more robust and to improve its reliability for our customers,” David Kubicek, the transit authority’s operations chief, said in a press release.