Also, your hypothesis is that a single person could kill everybody on a train, but in reality they'd get assaulted with suitcases, the doors between carriages would be barricaded, somebody would pull the emergency brake and people would flee, etc. This was actually tried once in France, and the attacker managed to kill zero (0) people before they were overpowered.
https://www.france24.com/en/france/20201217-thalys-train-gun...
There would be zero risk for the terrorist. There are no baggage check, and no one makes sure the owners of each luggage are on board. In theory each suitcase should have a label with the owner's name and address, but that too is not checked (and even if it was it would be the easiest thing to fake).
I just don't understand why it doesn't happen. I think the main reason is because there are very few actual terrorists, contrary to what everyone wants us to believe.
The amount of people per square foot surface, the fact that there is no way out and the fact that police cannot rush to the scene
Trains are left to themselves, there is no surveillance.
Ill intentioned people can change dynamic of the attack, instead of needing 2 planes and use them as kinetic projectiles against buildings, they'd need an armed guy on 10 trains
The end result in terms of human losses is the same because a guy with a gun can go through each and every compartment and off everybody before the police would even know what's going on.
Multiply that scenario by 10 or 20 or 30 and you have the worst national tragedy in the history of the nation.
I am thankful because if people on HN can't conceptualize that, then the odds of some goatfuc*er in the mountains of Afghanistan doing it are even lower....but still people thought they could not conceptualize the attacks happened in the 2000s. Everybody remembers the first, but there were also London and Madrid.