Trains are less of a threat. Attackers can only really cause damage on a train to a section and maybe a derailment.
Plane terrorism is broader. With a plane everyone is at risk and the plane could be used to attack other things.
Train terrorism is limited. A train can't be taken off course. A train can't be all blown up. Even a massive derailment only affects a section. An attack would be a tragedy but it has rails so to speak on the reach of the damage. Maybe if they blew it up over a bridge or by a building in a city it could cause more damage or some other trigger like causing another disaster. You to have the threat of people setting explosives on the track. There was one of those in 1995 in Arizona where 1 died and 78 were injured, supposedly the attackers were avenging the Waco Siege. [1] Looks like that is the only terrorism event on trains in the US. [2] Subways and metros are probably more targeted in the city.
[1] https://en.wikipedia.org/wiki/1995_Palo_Verde,_Arizona_derai...
[2] https://en.wikipedia.org/wiki/List_of_terrorist_incidents_in...
This is no way in comparison to train travel. There's no "dining car" on the train. There's no tables for business meetings. There's no 2 to 4 bathrooms per car. Sleeper rooms are much cheaper and accessible than on planes, but sure, you can pay a multiple of $10k for a room on beyond-first-class international flights.
But even if I granted you that in the old days, you could show up 30 minutes before the flight, walk pretty much straight to the gate, you're still ignoring much more time spent getting to the airport, and time spent waiting for takeoff and baggage loading/unloading.
Major airports are generally located outside cities and near water if possible, train stations are usually located in the heart of the metro area. Sure, there are exceptions like Las Vegas, but in general, most people have to spent 45mins-1hr just getting to the airport in the US.
No one's saying you should use train travel for DC to SF. But come on, NYC to DC, or SF to LA is a no-fucking brainer. When you drive from SF to LA, most of Route-5 is straight land on wild open, flat prairie, and the main difficulty is when you hit the mountain range past Bakersfield, which every advanced country, be it the French, Swiss, Germans, Japanese, Chinese, already deal with, on far more harsh terrain.
There's zero excuse for the opposition to high speed rail for regional intercity, except for a stubborn, weirdly political opposition to trains, which George Will kind of outlined (accidentally) in a piece attacking trains as "socialist" but somehow air travel as "non-socialist" despite the fact that the Feds and States have funded aviation infrastructure to the tune of hundreds of billions over the decades.