The blast occurred when leaking hydraulic fluid was ignited by welding equipment on a section of track near Union Station.
Two other contract workers were seriously injured and it was unclear how the incident would impact travel around the capital city.
The Washington Post said five downtown Metro stations had been shut down prior to the explosion as part of a major upgrade project. Work was halted Sunday and it was not known what areas would be reopened Monday morning.
The Post said weekend riders were being diverted to shuttle buses at the closed stations even before the explosion, and that ridership Monday would likely be down anyway due to the federal government shutdown.