1, 2, 3, 4, 5, 6, and Grand Central-42 St S trains were suspended in both directions.
President of NYC Transit, Andy Byford, said that the computers that keep track of trains failed, giving controllers no idea where the trains were. Byford says everything was shut down for safety reasons, and trains were instructed to stop where they were.
The MTA says a simple reboot brought the system back online but the root cause of computer failure is still unknown.
"This shouldn't have happened, New Yorkers should be able to rely on the transit system to get them home," Byford said.
Byford added that rescuing passengers was the first thing that had to be done, as well as identifying where the trains were.
Most of the trains had lighting, power and air conditioning.
Some passengers reported the suspension on numbered lines was causing a strain on other train lines.
Mayor de Blasio said the MTA owes every single New Yorker an explanation.
"This kind of meltdown during a heat wave is unacceptable," de Blasio said. "We've known about this dangerous weather for days. There's no excuse for why they aren't prepared."
NYC Comptroller Scott Stringer also called the situation "completely unacceptable."
Officials say the same issue stalled the train system on July 6.
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