So from all I have read it seems Plitt was filming a Superman themed sequence ("faster than a speeding bullet, more powerful than a locomotive") of him running in front of the train, tripped, fell, and was run over by the train and killed.
The crew claims that he tripped and fell, but apparently the surveillance camera on the Metrolink train and the video the crew shot shows that he simply got outrun by the train and probably didn't trip or fall. Some of the reports say the train just knocked him off the tracks and out of the way, so he didn't get run over. Even so, blunt force trauma of the impact and coming to stop on the ballast was enough to kill him.
I've been following this, and apparently a lot of his supporters seem to justify that he was somehow special and the normal rules shouldn't apply to him because he was so special. I haven't heard of one person claiming to be a friend or supporter saying that maybe he shouldn't have done this. All seem to rationalize the trespassing and just plain stupidity of performing a stunt like this on railroad property.