If you've ever caught a train in China there are airport-style security checks there - put your bag through the X-ray, walk through the arch, show your passport. In fact the whole experience is airport-style - you wait in a numbered lounge and only go onto the platform when your specific train is called.

Even taking the subway means passing through a quick bag check.

Airport-style security at train stations is a terrible idea for many reasons, but it's not as impractical as you make it sound.