Was it operator error or a technical glitch that caused a Metro Red Line train derailment in Silver Spring, Maryland, last month?

Preliminary evidence has yet to reveal the answer, but it is providing clues as to what caused the July 7 derailment along Colesville Road.

It shows the train operator moved the train past a red signal at the north end of the Silver Spring station platform, just as the Rail Operations Control Center was sending a command to move the switch associated with that signal back to a normal, or mainline, position.

“Due to the timing of the train passing the red signal, the lead car diverged from the main line, heading into the pocket storage track there,” said Dr. David Mayer, CEO of the Washington Metrorail Safety Commission, during a meeting Tuesday.

“The switch moved under the train derailing the lead trucks, the first wheel set of the second car,” he said.

Radio equipment glitches and other communication failures also plagued the response to the  derailment.

Mayer said they now have to sort through all of the evidence collected.

None of the 31 passengers were injured, but it took three hours to get trains moving again.