The Metrolink trains chug along at a pretty quick clip, around 70mph.
That's a metric shit ton of mass and inertia to stop.
The big heavy freight trains (2-3 miles in length, 55mph or so) can take upwards of 2 miles to come to a stop, and that's if the track was completely straight. Add curvature to the track and the braking time increases.
The Metrolinks are much shorter and lighter, but I would think it would take at least a mile for them to stop.
Any time you foul the tracks (ie: park a vehicle or equipment within 25 feet of the rail [freight rail standard distance, Metrolink may be different]) you need clearance and to have a flagman present at all times to act as a lookout.
http://www.nbclosangeles.com/news/local/Metrolink-Train-Crash-Oxnard-Camarillo-Ventura-County-293835911.html
The latest news report says it was a produce truck that was parked on the tracks and that the dude got out and ran away.
F*cking asshole.