Pepco Holdings corporate headquarters in Washington. (Jonathan Ernst/Reuters)

About 1,200 Pepco customers in the Union Station area were without power Thursday morning.

It is not immediately clear what caused the outage. There was no impact to train service or operations at Union Station, according to a Pepco spokesman.

Crews were on site and Pepco said it expects to have service restored by 9 a.m.

The outage happened around 6 a.m. when a transformer failed, Pepco said.