WASHINGTON — Metro is investigating how one of its empty trains rear-ended another overnight, sending two train operators to the hospital and significantly slowing the Monday morning rush. Now, Metro says the incident will have an impact on the evening rush, too.

Metro said that both passenger-less trains were bound for rail depots when they crashed just before 1 a.m. Monday between Farragut West and Foggy Bottom. The Washington Metrorail Safety Commission, the state-run agency now overseeing Metro, said both trains were on eastbound tracks when one train hit another that was stopped. The front and rear car of each train were “significantly” damaged, the safety commission said. Metro said the trains collided at a speed of 11 miles per hour.

The crash occurred in the transit system’s underground inner core, slowing trains along much of the Blue, Orange, and Silver lines to every 15 minutes on Monday morning.

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“Metro is encouraging all Orange, Blue, and Silver line customers to consider alternate travel modes during the PM rush hour,” said a statement by the transit agency. “Trains are operating every 15 minutes on each line, even at rush hour, and Silver Line trains are operating between Wiehle-Reston East and Ballston only. Significant delays and crowding are expected.”

“I've never seen the Ballston Metro this hectic,” tweeted Ashley Hopko at 9 a.m. Monday. Metro said it would refund delayed riders with a registered SmarTrip card as part of its Rush Hour Promise, but did not extend the offer to riders during the evening rush.

Metro said that train operators were released from the hospital by Monday afternoon. Metro said both operators were interviewed separately by investigators and screened for drugs and alcohol, a standard procedure after an incident.

“The investigation is also reviewing human factors as a possible cause,” said a statement Monday afternoon.

The Washington Metrorail Safety Commission said will that it will investigate “scheduling and possible operator fatigue or distraction, control center operations, signal and train control systems, vehicle integrity and maintenance to determine the cause.” Investigators will also look at on-board data about train speed and braking.

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This is the first significant collision of Metrorail trains in almost a decade. In November 2009, the operator of a six-car train hit a parked train in the transit agency’s West Falls Church rail yard. A news release at the time said three rail cars were damaged beyond repair.

In June 2009, one train rear-ended another train on the Red Line near Fort Totten, killing nine and injuring dozens more.

The pressure is on the transit agency to come up with a substantive answer to what happened.

“This morning’s commute on WMATA is a reminder that safety and reliability improvements still desperately needed,” tweeted House Democrat Gerry Connolly of Virginia. “My Subcommittee will have a hearing on this and other WMATA oversight matters on October 22.”

D.C. Councilmember At-Large Robert White said that “I expect significant investigation to determine the cause and how this will impact operations.”