This is amazing. It looks to me like the police officer's spidey sense was going off and that's why he sidled up next to him.
The Blaze reports:
A surveillance video posted to YouTube shows what is said to be an attempted suicide at a Medellin Metro station. A man is seen walking toward and away from the tracks looking for the oncoming train. Soon, a security guard or police officer comes into the camera’s view. Then, right as the train is pulling into the station, the man makes a leap toward the tracks, but the officer yanks him backward.