Toronto police say the male suspect was disturbing people on a northbound train Monday at around 4:30 p.m.
When a 28-year-old man asked him to stop, police said the suspect turned on him and attempted to rob the man. Police said a scuffle ensued, the suspect revealed a knife and demanded the man's personal property.
The suspect fled the train without taking anything from the man and was captured by security cameras as he left Bloor-Yonge Station.
Police didn't say how many people were on the train at the time of the incident.
Police describe the suspect as being in his 20s, standing six-foot-one with a medium build. At the time, he was wearing a black T-shirt, black baseball hat with the Wu-Tang Clan logo and black shorts.
Anyone with information about the incident is asked to contact police directly, or Crime Stoppers anonymously.