The incident happened Tuesday just before 2 p.m.
SEPTA says an initial investigation has revealed that a verbal argument ensued between the security guard and the 51-year-old man just before the attack.
According to the transit agency, the man seen throwing the punches works for Allied Universal providing security at the station.
Video shows the man punching the victim several times near his face as he was on the ground. The security guard then fled the SEPTA station after the incident.
The victim was hospitalized but is expected to survive.
"So there's a criminal investigation underway into this to look at what led up to this incident," said SEPTA spokesman Andrew Busch. "We need to interview the guard to get some more information from him to help us piece this together."
Officials say they haven't been able to locate the guard at this time. His account of the events are critical to the investigation.
According to SEPTA, the 51-year-old victim allegedly told detectives after the attack that "he did not remember anything leading up to the incident, or after the incident due to consumption of alcoholic beverages."
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SEPTA says it recently contracted with Allied to add an extra security presence, specifically along the Market-Frankford line. This is the first incident like this they've had, said Busch.
The guard will not be allowed to return to work on the SEPTA system.