Metro discovered cracks in two sections of rail yesterday, slowing the morning commute for thousands of riders on the Blue Line, but the transit agency said service should return to normal today.
Steve Feil, Metro's chief operating officer for rail, said it appeared that recent fluctuations in temperature caused the cracks. Feil noted that temperatures were as high as 70 weeks ago and that they had plummeted.
Dramatic swings from hot to cold cause steel to expand and contract, making it especially susceptible to cracking, Feil said.
The first crack, found at 7:09 a.m. at the Van Dorn Street station, was detected by an automatic sensor embedded along the railroad that signals when a breach occurs.
The crack was on the track running outbound toward the Franconia-Springfield station.
To get past the affected area, inbound and outbound trains temporarily alternated along a single segment of track. At 9:20 a.m., a temporary fix allowed trains to travel along the repaired track at reduced speeds.
Metro crews made a permanent fix yesterday afternoon that involved cutting out the damaged track and replacing it with new material.
Metro officials said a second crack on the Blue Line was discovered at 9:50 a.m. near the Reagan National Airport station, causing a second period of single-track operations.
Both cracks were vertical, which is the most common type of rail crack, Metro spokeswoman Lisa Farbstein said. The stretches of affected track were last inspected by ultrasound equipment in October and no flaws were found, she said.
Farbstein said she did not know the last time that the track walkers visually inspected the track except that it would have been within the past several days.
The crack at National Airport was discovered on the inbound track just south of the station. An automated signal alerted transit officials to the crack, Farbstein said.
A temporary repair was made and Metro planned to make permanent repairs after the subway closed last night, she said.
The affected rail at Van Dorn Street was installed in 1986. The rail at National Airport was installed in 1974, Feil said, and is among the oldest on the railroad. He said age might have contributed to that crack at the airport station.
Cracks have been discovered seven times in the past year, most of them on the Red Line.
In early October, a cracked rail at Fort Totten delayed service for about a half-hour. On Oct. 22, a 54-inch horizontal break at Judiciary Square forced a shutdown of the Red Line for 40 minutes during the morning rush, while workers searched for other problems. On Nov. 29, three cracks were found in the tracks between the Bethesda and Friendship Heights stations.
Metro's rail is inspected visually by track walkers and by ultrasound equipment designed to detect flaws too small to be seen.
Metro has increased ultrasound inspections from twice a year to five times a year and wants to intensify the visual inspections as well, Metro Chief Executive Richard A. White has said.
As part of his proposed budget, White wants to spend $9 million to fix an array of problems that have troubled the transit system this year, affecting public confidence and raising concerns about reliability and safety.
White's budget calls for $1 million to add 14 track inspectors to the 27 inspectors now authorized, which would allow more rigorous rail examinations.