Terrorism, mostly. Ever since they found plans in Bin Laden's compound to attack trains, there's been a ridiculous increase in rail safety and policing. Crews I work with at short lines are trained to report ALL trespassers to both local law enforcement & the local branch of the FBI. Not to mention with the increase in safety awareness, a trespasser can end up making the crew stop the train so they don't hurt the trespasser, which can result in hours of downtime plus throwing off the schedule for other crews, which can disrupt other railroads, which disrupts delivery times, etc. So one dumb trespasser can cause tens of thousands of dollars in lost time / late deliveries now.