Metro investigators think the recent heat wave caused Friday's Green Line derailment.

About 56 people were evacuated from a Green Line train Friday evening when it derailed near West Hyattsville just as it entered a tunnel.

Metro spokesman Dan Stessel said Saturday morning that investigators think the "probable cause" of the accident was a "heat kink," cause by high temperatures and prolonged exposure to sunlight. The heat causes the rails to expand and misalign, Stessel said.

"It is something that every railroad that operates in this kind of heat deals with," he said.

Washington-area temperatures have hovered near 100 degrees for nearly a week now; Friday's high was 98, according to the National Weather Service.

Metro's response to heat waves is to increase track inspections to be on the lookout for kinks. But there was no indication that anything was wrong with the tracks near West Hyattsville before the derailment, Stessel said.

After the accident Metro also ordered all trains to travel at only 35 mph on aboveground tracks. Green Line service near West Hyattsville will likely remain suspended for the rest of the day as crews repair the track, Stessel said. Riders can take shuttle buses around that section of track and can expect 30 minute delays.

No one was injured in the derailment, though one pregnant woman was taken to a local hospital as a precaution, Metro officials said.