[samatman]

To be frank about it, trains are a much softer target, since they can be derailed with an explosion on the track, or even a bit of welding.

Blowing up an airplane requires either bringing an explosive on board, or the use of a rocket-propelled grenade or full-on missile. Quite aside from the risks of hijacking mentioned in a sibling comment.

[NLips]

However, blowing up a bomb on a cruising plane will probably kill everyone. Blowing up a bomb on a cruising train will probably not - many people will be far from the blast and derailing trains are more survivable than planes breaking up at altitude.