Police said the incident happened near Yonge and Dundas streets at around 7:30 a.m. on Wednesday.
Investigators said the two transit workers were able to get away after they were chased by a suspect with a syringe.
There were no injuries to the employees.
The suspect was arrested at the scene, police said.
The incident comes after a rash of attacks on TTC employees recently. Up to 15 youths allegedly attacked two uniformed TTC employees on their way to work by bus Monday afternoon.