Officials are investigating what caused a Silver Line test train to derail Tuesday afternoon just west of Wolf Trap.
The train was traveling about 5 mph when the incident occurred.
Nowakowski said on Wednesday that this is the first time there has been a derailment since test trains began running. The first test trains began running the first phase of the rail line late last year.
Nowakowski said there are indicators to warn train operators that they are getting too close to the derailer, he could not elaborate on what they were in this case.
Metro officials said they had no comment because the incident is being investigated by the Metropolitan Washington Airports Authority.
In July, officials at the MWAA, which is building the $5.6 billion rail line, announced there would be at least an eight-week delay in substantial completion of the first phase of the project. That delay could affect the start of passenger service, pushing it into early 2014. The first phase will include five stops, four of which will run through Tysons.