Police say the 24-year-old woman went onto the tracks at the 14th Street-Union Square station just after 10:20 p.m. Monday.
Witnesses told officers she attempted to get back onto the platform but was unable to pull herself up.
She was struck by a northbound L train and pronounced dead at the scene.
Witnesses say the woman climbed down to the tracks, possibly to go to the bathroom.
The MTA advises riders not to go onto tracks, as it can be deceivingly difficult to get back onto the platform.
The agency also has lost item retrievers who can safely recovery personal items from the tracks.
L subway service was suspended for about three hours during the investigation.
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