Update 12:20 p.m. Metro crews have made a preliminary fix to the cracked rail
 on the Orange and Blue lines, allowing trains to resume running on
 both tracks. 
But the agency said residual delays could continue until about 1 p.m.
 Crews plan to return overnight when the system is closed
 to make a more permanent fix, according to Metro.

Original post: Metro riders are facing another round of delays due to a cracked rail on the Blue and Orange lines, the fourth crack in the past month.

The transit agency announced shortly after 11 a.m. Monday that a rail had cracked near the Rosslyn station, forcing trains to share a single track between Ballston and Foggy Bottom on the Orange Line and Foggy Bottom and Arlington Cemetery on the Blue Line.

The agency had no immediate estimate on when it could be fixed, but the break did not hit during the morning commute so the agency estimated delays of only about 15 minutes.

Last Thursday, a crack on the Red Line near Tenleytown caused major delays during the morning commute. The agency said it did not know the cause of that crack.

Two rails also cracked on Jan. 4 on the Yellow and Red lines, which the agency blamed on a sharp temperature change during a snap of cold weather.