A freight train and a MARC passenger train have collided outside Washington's Union Station, but neither train had passengers and no one was seriously injured.

D.C. fire department spokesman Pete Piringer says the trains "bumped" shortly after 9 a.m. Friday but did not overturn. He said four or five train operators were being evaluated but that their injuries did not appear to be serious.

It was unclear how the accident happened. Amtrak reported that service to and from Union Station is temporarily delayed.