The incident happened around 3:30 p.m. Tuesday at SEPTA's Race-Vine Station on the Broad Street Line.
Police say the suspect robbed a 16-year-old female at knifepoint, fled southbound at the City Hall Station and then took a Route 13 trolley westbound.
SEPTA police released a picture of the wanted man while he boarded the trolley at the West Plaza.
The victim did not suffer any serious injuries.
Police say the teen did notify police, but crimes like this one are often unreported.
SEPTA Police Chief Thomas Nestel said in light of recent crimes, he is hoping more people are learning they must report anything they see.
Last week, SEPTA made national news after riders on the Market-Frankford Line failed to call 911 or press safety buttons on the train while a rape was happening in progress.
In recent months, security guards contracted by SEPTA for a pilot program to enhance safety led to unprofessional conduct by guards, and the incidents were caught on camera.
Nestel believes the push to get more people to report suspicious activity and crimes on the transit system has increased already.
The pilot program with security guards is now being reassessed, but another pilot program utilizing social service specialists to handle the homeless population is now adding 57 specialists as we head into the colder months.
SEE ALSO: Train riders held up phones as woman was raped, SEPTA police say
"We immediately put it out so the public can help us," said Nestel when referring to images of crimes captured on surveillance cameras. "If the criminals are committing crimes on SEPTA and we don't know about it, we're not preventing it because we're not arresting them. (But), if you report it, we have 28,000 cameras, we're going to have video of that person, we're going to track that person down."
Anyone with information about the robbery suspect is asked to call SEPTA Police at (215) 580-8111.