The robbery was reported at about 8:35 p.m. near Third Street and Carroll Avenue.
The victim was sitting in the front of the Muni train when the two teens approached her and punched her, then stole the purses, which contained cash, a cellphone and other items, according to police.
The girls, described as about 13 or 14 years old, fled from the Muni vehicle and had not been found as of this morning, police said.
Anyone with information about the robbery is asked to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411 with "SFPD" in the message.
