Normal service has resumed on Metrorail after a heavy rail work vehicle derailed Tuesday morning on the Blue and Blue Plus lines.

It happened on tracks outside the Potomac Yard station in Alexandria, Virginia, when the equipment slipped off the track just before 5 a.m.

Trains single-tracked between Braddock Road and National Airport, with trains running every 18 minutes, causing delays during the morning commute.

Although the derailment did not involve a train, it caused delays because of speed restrictions in the area, the transit agency said.

The heavy rail work vehicle can drive on the highway and railroad tracks and was used to move equipment to the Yellow Line bridge work site, Metro said.