I don't quite understand the blather about the President wanting to control the populus. Whoever happens to occupy that office is under such a microscope they certainly couldn't control people even if they wanted. Ditch the political angle and look at reality.
We have people who want to terrorize us. That's a fact. Some of them are foreign - remember 9-11? Some are domestic - remember Oklahoma City? If someone had said, on 9-10-01, we'd better be careful or somebody may hijack planes and fly them into important buildings, you'd have thought they were nuts. In other words, just because it hasn't been done yet is no reason to dismiss the possibility.
Also, there have been so many assorted movies and TV programs about people putting bombs on trains, etc, that it's a no-brainer that some sicko is saying "hey, I could do that". Organized crime has used remotely-detonated explosives to eliminate rivals for decades - if somebody put something in a given railcar and pushed a button at the right (or wrong) time, there could be a catastrophe. Quite likely? maybe not. Very possible? Definitely!
The bottom line is that railfans have every right in the country to take photographs, watch trains, and so on, within the various laws about trespassing etc. When somebody is where they should not be, or when they are acting in a suspicious or unusual way, the safe and prudent thing is for the train crew to report it. As in many other aspects of our life, those who follow the law and use common sense won't have a problem. (Yes, I am aware we've had a few instances of rogue cops making up "laws" prohibiting photography - I'm talking 'under normal circumstances' here).