MARC Penn Line trains were stopped between Baltimore and Washington for just under two hours.
The Maryland Transit Administration said a person was walking on the tracks.
It happened near the Odenton station.
The woman was struck by a train on the railroad tracks south of the Amtrak Maintenance Center at 521 Old Waugh Chapel Road, according to the Anne Arundel County Fire Department.
The woman was transported to the R. Adams Cowley Shock Trauma Center in Baltimore with critical, possibly life-threatening injuries, the fire department said.
Trains were stopped around 7:30 a.m. and resumed before 9 a.m.
WTOP’s Colleen Kelleher contributed to this story.