The vast majority of trespassers are simply told to leave. Most departments just don't have the resources to make an arrest for petty misdemeanors, taking a patrol off of the road for hours to complete the booking process, write the report, and monitor the cell block for a prisoner who most likely won't even have the funds to be bailed on personal recognizance. Beyond that, Pan Am made a tactical decision to all but eliminate their police department. Regarding Lawrence, I would love to know what genius at the MBTA spent our money to put up fencing along the Merrimack Street section. There was a reason that the area hadn't been fenced in 176 years, the same reason that the fence is cut in at least 8 places already. You don't need a law degree to know that the liability of having a breached fence is much higher than that of not having one at all. I believe South Lawrence has the highest population density of any of Pan Am's yards, and there is little that can be done about the steady stream of pedestrians, especially on weekend nights. It certainly doesn't help that the yard is barely lit at night, and the equipment garage is somewhat less than secure.