The Los Angeles County Sheriff's Department Transit Services Bureau said the attack happened on a northbound Metro Blue Line train between 3:30pm and 3:45pm on Tuesday.
LASD Transit said 18-year-old Aliyah Von Lewis, of Long Beach, approached the victim and began hitting her.
In a video posted to social media, the suspect appears to put the victim in a chokehold and viciously throw her about.
The victim can be heard saying, "Get off me please."
The suspect appears to punch the victim more than 20 times.
In the video, a man watching can be heard saying, "I like that. I like that."
A second suspect, identified as a 17-year-old girl, then appears to punch the victim in the face.
A male in the video can be heard saying, "You better not call the police or I'm going to knock you out and take your phone."
LASD Transit said the male who said that is the 15-year-old brother of the second assault suspect.
In the video, the victim begins to cry and tries to walk away, but the second suspect appears to strike her at least 15 times.
The second suspect appears to pull the victim to the ground and knee her in the face multiple times. The video posted to social media stops shortly after.
LASD Transit said the train stopped at the Del Amo Station and the victim attempted to flee. But officials said a second attack occurred and more of the victim's property was stolen.
The victim reported the attack the following day, according to LASD Transit.
The video of the attack was posted to social media on Wednesday and the following day LASD Transit took Von Lewis and the 17-year-old girl into custody.
Both were charged with robbery, and assault with a deadly weapon likely to produce great bodily injury.
The 15-year-old brother of the second assault suspect was detained on Monday and is being charged with attempting to prevent or dissuade a witness from calling the police and assault with a deadly weapon likely to produce great bodily injury.
The juvenile siblings were taken to Los Padrinos Juvenile Hall.
Von Lewis is being held at Century Regional Detention Facility.
LASD Transit stated more arrests are possible in the case.
Anyone with information about the attack is urged to call authorities at (800) 222-8477. Callers can remain anonymous.