A train derailed in a Metro rail yard over the weekend, crashing into another train and damaging three rail cars plus other equipment.

No one was injured, but it was the latest safety problem to occur on Metro's rail system. The chairman of the Tri-State Oversight Committee, which monitors Metrorail's safety, said the incident was concerning.

The train had no riders aboard it at the time and was in the New Carrollton rail yard in Prince George's County, where trains are stored and maintained. It was in the process of getting more cars added to the middle, known as bellying cars onto the train, when the incident occurred about 1:50 a.m. Saturday, according to Tri-State Oversight Committee Chairman James Benton.

It appears the problem stemmed from human error, though the investigation is continuing, Benton told The Washington Examiner on Tuesday. Two employees have been put on paid leave while officials determine the cause, according to Metro. The news of the derailment was first reported by the blog Unsuck DC Metro.

The problems appear to have occurred when the train started to reverse through a switch, where the rails diverge, Benton said. One portion of the train went in one direction and the other went onto the other set of rails, striking an empty parked train with its rear car.

The train was traveling at a low speed when it derailed, according to Metro spokesman Dan Stessel, but the track, the third rail, the switch and three rail cars were damaged. Stessel said it was too early to know how costly the damage would be but said it did not appear to be as high as $7 million, an estimate reported by Unsuck DC Metro.

Benton said the cars had to be lifted with a crane, which is expensive on its own.

The track equipment was repaired over the weekend without any effect on train service, Stessel said.

Both Metro and the oversight group are investigating the incident. The workers are slated to be tested for drugs and alcohol use, as is standard after any incident, Benton said, and safety officials will review their recent work history to determine if fatigue played any role.

In July, a Green Line Metro train carrying riders derailed near West Hyattsville after the rail bent in what is called a heat kink. A Blue Line train derailed near Rosslyn in April after what Metro called a human error when a technician failed to check whether a clamp was properly in place on a track switch. Another train derailed near Farragut North in February 2010 after an operator ran a red light in a pocket track.

Other rail yard derailments occurred in March 2010 in the Brentwood yard and the West Falls Church rail yard in November 2009.