UPDATE 6:33 a.m. 7/16/2015: Metro says that a test train reports good track conditions, and the Red Line is no longer single-tracking.

EARLIER: WASHINGTON — A “slippery rail condition” caused significant delays on Metro’s Red Line Wednesday, and the problems have persisted for nearly 24 hours.

Trains are single-tracking between Twinbrook and Grosvenor Thursday morning, as they have been since Wednesday morning, and Metro announced at about 6:20 that outbound Red Line trains will bypass the White Flint and Twinbrook stations.

Metro’s Dan Stessel said Wednesday that an oily substance was found on a track outside the Medical Center station, and was tracked by trains’ wheels toward Twinbrook. On Thursday, Metro added that the slippery rail is the result of the rupture of a foam-filled tire on a piece of construction equipment on the outbound track in the Tuesday-Wednesday overnight.

Riders have been warned to expect delays and overcrowding. Repair work continued through Wednesday night into Thursday morning, and a test train will run early Thursday to determine whether and when normal service will resume.

Get the latest updates on Metro’s website and tune in to WTOP Radio on the 8’s to get updates as well.