People have jumped from public places before like balconies and of course, Beachy Head, so I don't see how Niagara Falls would be any different. That being said, I understand the concerns about not wanting to traumatize bystanders.
In terms of the train, you'd be placing someone unassociated with you in a position where they are directly involved with your suicide process. Jumping off Niagara Falls doesn't have that, as any people around would not be actively participating.Jumping is a fast way to go, and it really depends on how high you fall from. Most bystanders won't be too shaken up since it all happens so fast, but if you land right next to someone, they might get pretty freaked out. I've seen a bunch of videos where this happens, and usually, the person just bolts and calls 911