If you fence off all the right of way they'll probably try using station platforms and streets where they can walk around the gates.

I knew someone who had pancreatic cancer and killed himself by walking in front of a Caltrain a few years ago.
I believe he kept his feelings and intentions to himself so he was able to skip work one day and go down to the tracks.
I'm sure there are lots of others who keep their thoughts private and don't seek help so it's next to impossible to prevent suicide on the tracks unless you have cameras covering every inch of the right of way and people close enough to get them out of the way.
I believe Caltrain has some cameras near the right of way in Palo Alto near schools where students have committed suicide.