Some thoughts I just had reading these comments: We all accept that boarding a train is a much more pleasant experience than boarding a plane, largely because you don't have this need for massive security waits, arriving three hours early, etc. But... why don't we have those things on trains? I can think of a couple reasons: 1) trains tend to be mostly used for domestic travel (in North America), so you don't have border checks slowing things down, and 2) trains can only go where the tracks go, so although you could certainly carry out an attack on a train, it would be much more difficult to use a train as a weapon against something else, or to try to stage a getaway of some kind by taking a train off course.
Perhaps there's only so much that can be done about #1, but regarding #2, I wonder if we could rethink airport security once we have completely autonomous planes. If the plane is controlled completely by autopilot and/or remotely, it's basically on tracks, so I can't see any logical argument why we couldn't then take the same approach to domestic flights as to domestic trains. (You could argue that that's already the case with locked cockpit doors, but there's an argument on both sides there.)
There are other differences, e.g. one might hope to escape off of a moving train or arrest its progress, both of which are nigh-impossible on a flying object, autonomous or not.
Perhaps there's only so much that can be done about #1, but regarding #2, I wonder if we could rethink airport security once we have completely autonomous planes. If the plane is controlled completely by autopilot and/or remotely, it's basically on tracks, so I can't see any logical argument why we couldn't then take the same approach to domestic flights as to domestic trains. (You could argue that that's already the case with locked cockpit doors, but there's an argument on both sides there.)