Greg Plitt Dead -- Bravo Star Dies ... Struck By Train

5:45 PM PST: Cops say two men were filming Plitt on the tracks when the train conductor spotted him in the way. He attempted to stop and began sounding his horn -- but Plitt remained on the tracks.

Investigators seized video of the incident as evidence.

Fitness instructor Greg Plitt -- who appeared on the Bravo show "Work Out" -- died Saturday after he was struck by a train.

The 37 year-old was hit by a Metrolink train in Burbank around 4PM ... and according to KABC he was with some friends who may have been shooting video at the time.

One passenger reportedly said Plitt had a camcorder in his hands when he stumbled over the tracks.

Law enforcement tells us they believe Plitt was shooting a workout video at the time.

The incident is under investigation.

Plitt has modeled for Calvin Klein, Old Navy and Under Armour ... and images of his body were used to help create the physique of Dr. Manhattan in "The Watchmen."