There was the same discussion a few days ago on another post. It's not a fair comparison. You can hardly hijack a train. Even if you manage to get control of the train, you cannot land it in a tower full of people. The worst you can do is make it de-rail which, while it will probably kill a lot of people, is unlikely to kill all of its passenger. And if you carry a bomb you will blow at most one compartment. Also, with train, even if someone manage to get to the driver compartment, you can still control a lot of the train from the outside. You can change its direction at exchange point, you can cut its supply in electricity (if it is an electric train) therefore stopping it (although in quite a dangerous way...). And you are still on the ground were it is very easy for police / military to catch up and intervene. You can't do that with a plane several kilometers high in the sky.

Planes are much more sensible to all of those potential threat. That is why we ended up with such drastic security. Although I have to say, in a lot of places, security is way more relaxed. If you travel from the E.U to the E.U you have way less check and queue time.