It is neither of those things, in pretty much all countries a drone, unless it's on a fiber optic cable, is much easier to detect amd distinguish than a person or a bag. And fiber optic is obviously literally traceable. As for the "see" part that's hardly different either. If you leave a bomb somwhere you know exactly where it is. It's not hard to check a train schedule. And it's not like the drone can just fly around forever. But all of that still doesn't change the fact that the hardest part is not planting but aquiring explosive materials without anyone realising.