Metro trains slowed Tuesday in above-ground sections due to the heat, the transit agency said.
The decision to slow trains to 35 mph is made based on several criteria, including the temperature of the rail. Actual high temperatures were around 100 degrees Tuesday and will be again Wednesday. Highs are expected to be in the mid-90s on Thursday.
Tuesday's speed restrictions went into effect at 2:30 p.m. Trains resumed normal speed at 6:30 p.m.
The speed restrictions could be implemented again Wednesday. Riders will be allowed to drink water in the system Wednesday due to a heat advisory.
Metro said inspectors measure rail temperatures in numerous ways, including via a thermal gun. High temps and direct sunlight on rails can cause them to reach 135 degrees or higher, according to WMATA. If they reach that temperature, a "slow order" will be put into effect.
"While these speed restrictions may cause some delays and inconvenience, they are intended to put the safety of our passengers first," said Dave Kubicek, Metro's Deputy General Manager for Operations, in a release.
Metro Investigators determined that a so-called "heat kink" was the "probable cause" of a derailment that snarled mass transit during rush hour on July 6.
Three cars of a southbound Green Line train derailed at approximately 4:45 Friday afternoon as the train was entering a tunnel from an outdoor section of the tracks near the West Hyattsville station. No injuries were reported as a result of the derailment, but 55 passengers were evacuated and a pregnant woman was taken to the hospital for observation.
A heat kink can occur due to the expansion of steel rails during extremely high temperatures, according to Metro. If the force of an expanding rail cannot be constrained by the ties, clips or ballast, a sudden release of pressure can cause the rail to move laterally, resulting in a "kink."
The speed restrictions would improve a train's ability to stop in the event that a heat kink develops on the rails, according to Metro.