The ones I've run into have been in cars. As I've been given to understand, "abandoned" and "unused" are not the same. The Right of Way is supposedly only abandoned if the RR removes rails and signs off their rights and (usually) turns the ROW over to a unit of Gov't., usually as a "linear park" or rail-trail. My understanding is that until officially abandoned, they have the right to patrol or inspect any RR lands. That's how they apply the law here in MI. YMMV.Are these RR police using rail cars to patrol? If so, the should be fairly easy to avoid. Do they patrol abbandoned lines?
Abandoned tracks can get really bad quickly. It is the bridges you really have to watch out for. As far as running a vehicle on old tracks you would need to have one that the tires line up exactly with the width of the tracks. YOu can deflate you tires just a bit and then you should be able to run on the tracks. Problem is you have to be able to reinflate your tires once you choose to get off. Go slow since the switches can be thrown and you will not know it till you get to the switch itself. I am speaking about siding cutoffs. We did it as kids and were lucky we did not get killed.