It happened before 1 a.m. on March 16.
A METRORail spokeswoman said the suspect was harassing a young woman on a train headed south from McGowen station at first, but then turned his attention to the 62-year-old man who boarded the train.
The man first verbally, then physically assaulted the victim, authorities said.
ABC13 spoke with one passenger, Jon Oxley, who believed he witnessed the tail end of that incident.
"The guy doing the assaulting is one of these that does the assault and runs for the door," said Oxley.
According to METRO, a social media tip led to the arrest.
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