Friday's derailment of three Green Line train cars was heat-related, investigators say.
The misaligned rails are known as a "heat kink," which happens when sections of the metal rails expand due to "extreme heat or prolonged exposure to direct sun."
No injuries were reported.
Prince George's County Fire Department says the incident occurred around 4:45 p.m. A pregnant woman was transported to a local hospital as a precaution.
The fire department says 55 passengers were removed from the train. They were removed via a fan shaft exit at 29th Avenue and Ager Road.
"Metro has imposed an immediate 35 mph speed restriction on trains operating in all above-ground territory." Metro reports. "As a result, Metrorail customers will experience delays on all lines."
All previously scheduled track work for the weekend has been canceled.