Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
All that can be said about this saddening film is that this ain't your average zombie thriller.
- Seok-woo's death is nigh-objectively the saddest part of the movie. He gets infected by Young-suk, the most despicable character in the film, and he stays human long enough to throw him off the train car and give a final goodbye to Su-an before staggering to the other end of the train car while Su-an cries out and begs him to stay. At this point, he breaks down sobbing and, at the exact moment that he turns, thinks of holding Su-an as a baby for the first time, which gives him enough peace to throw himself off the train car, smiling all the way.
- The music that plays over this moment.
- Shortly before, the expression on Seok-woo's face when Su-an begs him to stay with them. This is the face of a father who realizes that his daughter does love him more than he knew. And that's all the more reason to keep her safe from himself.
- And after this, we get a shot of the train chugging forward through the idyllic South Korean landscape. No blood, no smoke. Just a countryside visibly untouched by the outbreak—a sharp, deliberate contrast to the gory, tear-inducing majority of the film.
- The ending, where the emotionally-wrecked and physically-exhausted Seong-kyeong and Su-an are walking down the final tunnel. Just as it seems that soldiers are about to shoot them, their not being zombies is proved by Su-an singing "Aloha 'Oe", the song she failed to finish singing at the recital as Seok-woo wasn't there, which is now repurposed into a memorial to her father.
- Sang-hwa performing a Heroic Sacrifice and asking Seok-woo to take care of his wife, who is looking on in tears. And to make it even more heartbreaking, Sang-hwa finally gives Seong-kyeong a name for their baby—just before the zombies smash the glass, and Seok-woo has to drag the despairing parent back.
- Jin-hee becoming infected and infecting an emotionally-wrecked Young-guk, who is so distraught over Jin-hee's fate that he doesn't try to save himself, instead crying and holding her until she turns and bites him.I'm sorry...!
- Just the look of pure despair on Jong-gil's face as her sister In-gil sacrifices herself by staying behind so Seok-woo can close the door rather than risk the infected getting in while Seok-woo tries getting to her. The fact that In-gil smiles at her sister just a mere moment before the infected grab her only makes it all the more heart-wrenching.
- Another tearjerker is after the sacrificing sister gets infected. As the zombies are trying to break through, they all look like the ravenous monsters they've been throughout the movie. That is, except for the newly-turned In-gil. She's not another blood-covered monster yet. She just looks confused, lost, like a child.
- The woman who brings the virus onto the train apologizes over and over again, crying, as she desperately tourniquets herself in a futile attempt to stop the infection.
- The homeless man sacrificing himself to allow the others to escape.