This story is a good example of one major reason: trains are way more robust. Imagine a similar catastrophic fire in the cockpit of a high speed train. Worst case, you hit the emergency stop and then evacuate the train. The only person who would be at risk would be the train driver. Have that fire in an airplane and everyone on board dies.
This incident wasn't a terrorist attack, but the same idea applies there. The Lockerbie bomb, for example, was pretty small. Setting it off in a train might have killed some nearby people, but that’s all. But set it off in an airplane and you can kill hundreds.
You don't even need to do that, because most trains and pretty much every high-speed train has some sort of dead man's switch, so the driver leaving their seat would automatically enable emergency brake in short order as they would if the driver had e.g. fallen unconscious.
You can (usually) stop a train within less than a minute and then trivially evacuate it.
They're also less dense than most other public transport, including planes.
And it's impossible to add security to local transit, because standing for 15 minutes at a security checkpoint for a 15 minute bus ride will make everyone get a car and/or unelect the idiot who implemented that rule.
2 dead (both United Pacific employees, I assume train crew) and 3 light injuries from a train crashing into a truck, derailing, and hitting a building I think is pretty good evidence that using a train as your kinetic delivery is not a great way to do terrorism.
In all those examples trains are no different than movie theaters, malls, markets, sport and music events, etc. i.e: just places where large groups of people gather.
Another reason is, security checks are just one part of the whole process involved. A flight with 100 people dying because of a bomb may be more “glamorous” than 200 dead on a train to a terrorist organization.
Also: the actual number of such people/organizations, is, fortunately, extremely low in daily life.
And what do you want to do with a high speed train?
They are equipped with a dead man switch, and will shutdown pretty much immediately if not responded to in time. Case in point: the high speed train driver that committed suicide by jumping out of his train two days ago. (May he rest in peace.) And ultimately, they are computer monitored, and will be shutdown if a given set of security procedures are not respected.
French trains have had bombs explode in them in the past, and did not derail. Some have hit landslides at full speed, and did not derail.
High-speed trains are fine, no need for airport-like obnoxious levels of "security".
This incident wasn't a terrorist attack, but the same idea applies there. The Lockerbie bomb, for example, was pretty small. Setting it off in a train might have killed some nearby people, but that’s all. But set it off in an airplane and you can kill hundreds.