I seriously doubt that Amtrak has to worry. Any operative is going to have a diffcult enough time, he's not going to waste precious resources fiddling around in Trinidad Colorado. Maybe a big city, but then a commuter train, not a passenger train, as in Madrid. These guys go for shock and awe, too. And an Amtrak coach blowing up in the middle of Iowa will do zilch for them.
I'd put the bomb-sniffing dogs on the commuter trains, at random.
No way to watch all the people on all the trains all the time. That's a fact.
Perhaps our government needs to start looking at root causes of terrorism instead of trying to fight them on the battlefield.