I have NO love lost for security, but consider this:

On a plane, you can make it go anywhere. You can fly it to NYC and crash it into a building, or you could fly it to a packed sports arena and crash it into the crowds. Either way you can kill many people.

On a train, you can only go on the tracks. You could possibly keep on the gas and crash into the terminal or run it so fast it goes off the tracks, but you can really only ever endanger the people ON the train, and at that you can really only endanger them by causing an accident to the train. The most effective thing to do would be to bring a bomb onboard and blow it up in the tunnel, costing billions to repair and (if the blast it strong enough which is highly unlikely) causing the tunnel to flood and killing a lot of people in it.

But with an Axe, what can one person do? At most they can commandeer the train (again, unlikely) and - what? drive it real fast? They probably have max speed limiters and auto-braking systems that prevent them being run off the track and being crashed into the terminal. So what can someone do to cause death and mayhem by commandeering the train? Not much.

So, I would submit that the major focus of train security would be explosives, and therefore it was probably more effective to concentrate on possible explosives rather than possible tools that could be used to take commandeer the train.

My .02 - not that it makes the situation any better/safer/clearer