Actually, whether he was near any of the signs or not or even saw any of them is irrelevant. Those signs are evidently spaced sporadically around the platform, including poles which boarding or disembarking passengers have to pass in the course of getting onto or off of the trains. No ticketed passenger would logically assume those signs applied to him since he has permission from Amtrak or whatever railway he is traveling on. Would YOU assume they applied to YOU if YOU saw them when you got off of a train? Those signs are aimed at people who have no valid reason to be on the platform. Ticket passengers certainly have a valid reason to be there.

Actually, that "security guard" was an employee of Jones LaSalle, the management company which runs the "mall" portion of Union Station. They have since been told, in no uncertain terms, that they cannot interfere with photography anywhere withing the public spaces of Union Station. They only operate the "mall" and they do not "own" it. The US Government, and thus all of us, own it.