Authorities say Dava Denise Webster abandoned her 6-year-old son at Union Station on July 4 and walked away.
Police had trouble identifying the boy, who did not communicate with investigators.
They say Webster returned to Union Station the next day, where she was recognized and taken into custody.
Webster appeared in court Monday afternoon and entered a not guilty plea. She is due back in court July 20.
If convicted, Webster faces up to six years in prison, according to the Los Angeles County District Attorney's Office. Prosecutors are asking she be held on $100,000 bail.