Yes, terrorists can also travel in trains. And here it seems to me to penetrate them much easier than on an airplane.
Actually, this raises interesting point.
How common are train attacks? Once per decade? Vast majority of humankind travels by public transportation system, not by planes. Yet, we are almost never witnessing these attacks.
It might be, that planes are inherently more high profile targets. Even one man with grenade can kill everything aboard a plane and the news about it will go around the world. Derailed train? Nah.
Aircraft are still more popular for committing terrorist acts, because they are faster and they are used by many influential people who can be targeted.