Generally yes, would be safe for solo women, but depends on the line and neighborhood. Blue, yellow, orange, and silver lines are mainly in safe areas, though it can get worse once you get to the east of US Capitol. Red & green are somewhat worse, so be more alert on those, especially in the northern parts of the city.
It’s smaller than the NYC subway since Washington is much smaller, and also goes outside the city to suburbs in MD & VA. Also much cleaner and likely safer than equivalent neighborhoods’ subways in NYC.