A Metro train derailed at the East Hyattsville Station in Prince George’s County. (JUANA ARIAS/FOR THE WASHINGTON POST)

A Metro train carrying about 50 passengers derailed Friday in Prince George’s County on a steaming-hot afternoon, the second time in three months one of the rail system’s trains jumped the tracks with riders aboard.

No injuries were reported in the incident on the Green Line near the West Hyattsville station. But it came at a time of continuing concerns about the safety of the system, which carries 700,000 passengers a day and is the second busiest in the nation.

The incident represented another challenge to public services across the Washington region, which have struggled with the effects of record heat, severe storms and a crippling loss of electricity.

The official cause of Friday’s derailment was not given. It happened about 4:45 p.m. as the six-car train, headed toward the District, approached West Hyattsville.

However, it appeared that the 99-degree heat may have been a factor.

'The scene at L'Enfant Plaza going toward Branch Avenue/Huntington' - tweeted by @kmdaniels12 (Katharine Daniels/Katharine Daniels)

“It is something that we’re going to be looking at as part of the investigation,” Metro spokesman Dan Stessel said.

Heat expands metal rails, which can produce kinks or bends on outdoor sections of track. Orders to reduce speeds are sometimes issued in such conditions, and after the derailment, all Metro trains were ordered to stay below 35 mph on aboveground tracks.

Television footage, taken from a helicopter and broadcast on WRC (Channel 4) showed an outdoor section of track that appeared to be buckled and about six inches out of alignment. The section appeared to be about 500 feet from the spot where trains go underground between Prince George’s Plaza and West Hyattsville.

Stessel said the entire track system is inspected twice a week and that because of the temperatures more workers had been walking the tracks. He was unable to say when the area involved was last checked.

The train came to a halt inside a tunnel at a point about 1,000 feet from West Hyattsville. Three cars derailed — two 5000 series cars and a 1000 series car, Stessel said. All three cars on the six-car train remained “inline and upright,” Stessel said. According to a television account that could not be immediately confirmed, the cars may have come to rest against the side of the tunnel.

Within 15 minutes of the derailment, Prince George’s firefighters reached the tunnel, shut off power and began evacuations. At least some of the passengers were led to a ventilation shaft that also serves as an emergency exit.

“It went pretty easy,” said Earlene Thomas, who watched from her apartment. “No one looked in bad shape.”

Video taken inside the train after the derailment and shown on WRC indicated that calm prevailed as passengers awaited evacuation. But the situation was also described as frightening.

Based on passenger accounts to WRC, it appeared that the train shook before halting.

At least one passenger praised rescuers. They were “well organized,” the passenger told WRC, estimating that the evacuation s took 20 to 25 minutes.

Authorities said all passengers were out by 5:40 p.m. They added that a pregnant woman was taken to a hospital for an exam and that her condition was described as good.

The train’s speed was unclear. The tracks curve sharply to the left between Prince George’s Plaza and the tunnel entrance.

The 5000 series rail cars have been problematic. The cars, made by CAF of Spain, have a history of electrical, software and wheel issues, including a problem with derailments. The Washington Metropolitan Area Transit Authority spent $383 million on the rail cars, which were delivered to Metro between 1998 and 2003.

After the derailment, Metro called off track work planned for this weekend on all lines. Green Line service was suspended between Fort Totten and Prince George’s Plaza. Metro said it was providing bus service.

On Friday evening, a track problem that interfered with service was reported in the Suitland area of the Green Line. Details were not immediately known.

The most recent reported derailment of a train carrying passengers was in April near the Rosslyn station. It apparently was caused by a problem with the positioning of a switch that transfers trains between tracks. Switches and sharp curves are typically most vulnerable to derailments.

Heat was blamed for at least one serious Amtrak derailment in this area when a passenger train left the tracks in 1992 in Kensington, toppling six cars and injuring 97 of 176 people aboard.

Overall, watchdog and public-safety groups said last month, Metro is safer now than at the time of the fatal Red Line crash near Fort Totten in 2009. But system officials have said they have not reached all their goals.

Clarence Williams contributed to this report.