Some of those were at train stations, not in trains. The same type of terrorism would work any place you find dozens or hundreds of people in close proximity. The whole reason the terrorists in all these incidents targeted trains or train stations is because a somewhat-large number of people were gathered there.

If you want to destroy skyscrapers (and maybe kill thousands or tens of thousands in the process), and not just murder a few dozen people, train-related terrorism isn't going to work for you.