(Reuters) - Several cars in a Metro train derailed in a Washington, D.C, suburb on Friday but none of the 55 passengers on board were injured, and investigators will look into whether heat was a factor in the accident, a transit spokesman said.

The three cars that derailed in a tunnel in Hyattsville, Maryland, stayed upright, said Philip Stewart, a spokesman for the Washington Metropolitan Area Transit Authority.

A pregnant woman was taken to a hospital as a precaution, Stewart said, and other passengers were put on a bus.

The cause of the derailment has not been determined, but extreme heat may have been a factor, Stewart said.

Heat can affect rail tracks, he said, and the temperature in Hyattsville was in the high 90s on Friday.

The train that derailed was a Green Line train from Greenbelt, Maryland, to Washington, D.C.

(Reporting By Alex Dobuzinskis; Editing by Cynthia Johnston and Todd Eastham)