Happened in France, fortunately with no casualties...

Not taking this into account is immoral, because those resources could potentially be spent saving peoples lives in more efficient ways. Terrorism is extremely rare way of dying.

I'm extremely skeptical that this accounting will even show a benefit for airport security, but I don't think airport security is mainly about stopping terrorists. It's about controlling what people bring on board the plane, as there's a lot of things that could inadvertantly cause trouble on a plane. The same can not be said for trains to the same degree.

Also, planes themselves can be used as effective weapons if taken over. Don't think that goes for trains, especially where they're electrified.

It continues to be cool for the thousands of other trips that day and the millions of other trips afterwards where that doesn't happen.

Better to build a society that doesn't create people who feel the need to mow down a train car full of people than to treat everyone else like a criminal a priori.

This is the case for any crowded public space. It is unavoidable.

A simple metal detector would've caught a long gun, even with incompetent security personnel. By only allowing small "carry on" bags in the passenger carriages (storing larger "checked" bags in a separate freight car) the attack space is limited to what can fit in a handbag or backpack. It won't catch everything, but wrt terrorist attacks trains are safer than airplanes.

You don't need to go full TSA and make grandma take off her shoes or racially profile people to have decent security.