People might get themselves stuck on the wrong side of the door.
and push a button on that side of the door to open it.
but then you can sneak in between the train and the walls and then commit suicide.
Since there would normally be no one there except when a train is present, it should be easy for whoever monitors the security cameras to spot such people.
A large portion of Caltrain is at ground level with streets, such as at the station at Mountain View. You could easily walk around such barriers by walking on the road and into the wrong side...