The new switch was installed outside Prince George's Plaza station on the Green Line.
"Guarded 8s" are guarded rails that permit trains to move between tracks more safely, thereby decreasing the risk of derailment.
"The aggressive program to complete the installation of the Guarded 8 switches throughout the rail system demonstrates Metro's goal of making safety a top priority while moving Metro Forward through our rebuilding efforts," said Richard Sarles, WMATA general manager and CEO.
The $57-million project cost was funded by the federal government and WMATA's local jurisdictions.
The NTSB recommended the installation of Guarded 8s in 2007. Between 2008 and 2012, WMATA installed 108 of the recommended 178 switches, including 64 in past two years.