The kink in the track where Green Line trains derailed Friday may have been caused by the heat wave plaguing the D.C. area. (WMATA)

The heat wave of the past week has been hard on Metro, taxing the system’s equipment and employees, and on Saturday, transit investigators said the excessive temperatures were probably to blame for the derailment of a Green Line train Friday afternoon in Prince George’s County.

The 99-degree weather apparently caused a “heat kink” in the rail near the West Hyattsville station, derailing the inbound train and raising questions about how effectively Metro is monitoring its infrastructure for hazards brought on by the heat.

Since the derailment, Green Line service between Fort Totten and Prince George’s Plaza has been replaced by shuttle service but is expected to resume in time for Monday morning’s commute.

Heat kinks are a well-known phenomenon in the rail industry, and Metro, like other transit systems with aboveground tracks, is responsible for keeping a close eye on its tracks during periods of sustained high temperatures.

While the agency can anticipate when heat kinks are likely to occur, it is not as easy for it to predict their location. “They happen very suddenly,” said Andrew Kish, a rail safety consultant who is an expert on track stability.

“They get unstable, they buckle out laterally. There’s no warning, generally.”

Metro’s procedures call for every section of the system to be checked weekly and for above­ground tracks to be checked more frequently when the temperature exceeds 95 degrees.

Dan Stessel, a Metro spokesman, said he did not know when workers had last checked the stretch of track where the derailment occurred. Stessel said that would be determined during Metro’s investigation into the accident.

The District-bound train was carrying 55 people when it derailed about 4:45 p.m. near the West Hyattsville station. No injuries were reported, and rescue workers evacuated the train in about an hour, leading the passengers through a nearby tunnel and out a ventilation shaft that serves as an emergency exit.

It was the second time in three months that one of the rail system’s trains jumped the tracks with riders aboard, and it was the second time in a week that a disabled Green Line train had to be evacuated in Maryland.

Since the Red Line crash that killed nine people in 2009, Metro has worked to make its operations safer, and regulators have reported improvements.

But concerns persist for the system, which carries about 700,000 people a day and is the nation’s second-busiest subway system.

Recently, questions were raised about Metro’s failure to ensure that emergency exits were accessible. And in May, a worker in the train yard in Shady Grove was gravely injured after being struck by a train.

Heat kinks such as the one that occurred Friday have surfaced before on the Metro system, but even enhanced inspections can not prevent every mishap, Kish said.

Excessive heat in July 2010 led to a kink on the Red Line near the Rhode Island Avenue-Brentwood station; Metro slowed its trains and replaced the problematic rail. In 2011, there was another kink on the Red Line, this time between Takoma and Silver Spring, which Metro also corrected before any of its cars could run off the tracks.

Aboveground tracks are at a much higher risk of warping because they are exposed to direct sunlight in addition to the warm air.

If a backyard thermometer reads 100 degrees, an above­ground Metro track could be as hot as 140 degrees.

But according to Kish, the recent heat wave is only half of the heat-kink equation. Kinks are dependent on two factors: the air temperature — which has been hovering around 100 degrees in Washington for the past week — and how well a stretch of track can handle stress.

While rails typically expand in the heat and contract in the cold, methods have been developed to minimize the hazards of such changes.

Track specialists have characterized rails by their so-called neutral temperature, a kind of base temperature at which they are least prone to expansion or contraction. But after long exposure to both heat and cold, the neutral temperature may change, making the rail more vulnerable to fluctuations in weather.

As it turns out, the stretch of track that buckled Friday was scheduled for routine maintenance in the coming week. Instead of planned third-rail improvements, that track is getting a more thorough makeover this weekend.

The National Transportation Safety Board often conducts its own investigations after a derailment, but NTSB spokesman Keith Holloway said that because there were no fatalities and there is a relatively clear idea of what happened, “it doesn’t really merit an NTSB investigation at this point.”