I think it is difficult to make an original zombie film, given that more or less they all present the same clichés of the genre: unfortunately even this (although pleasant) South Korean film cannot avoid the feeling of "already seen", especially for the story in general and for the creation of the zombies, very similar to those of World War Z in terms of movements, to the point that one almost has the sensation of seeing an oriental spin-off of the film with Brad Pitt, with the aggravating circumstance of not offering any context and no explanation of what is happening (also because, obviously, the information on the characters' cell phones is in Korean). Paradoxically, however, the film is completely understandable even without any information because it is so inserted into the tracks of the genre (just to stay on the subject of trains) that there is no need to explain anything: the viewer already knows that people are going crazy due to some virus that revives the dead transforming them into zombies hungry for human flesh who will organize a kind of revolt, and many regards to common sense and logic.
The characters, apart from the protagonists, are poorly characterized and mostly reduced to archetypes, if not to actual sketches (the strong hero, the teenagers in love, the selfish businessman, etc.) , also as a result of a somewhat caricatured acting. Also for this reason the film almost resembles a video game, and as a fan of this last form of entertainment I say this without intending to diminish the film, but simply to convey the idea of ​​some moments a little far-fetched, especially in the central part. (and it takes a lot to be unbelievable in a film that talks about the zombie apocalypse).
An element of originality is given by confining most of the film to a train, transforming the protagonists into prey locked in cages with their predators. However, even this considerable potential was not, in my opinion, properly exploited: I never perceived the sense of claustrophobia and real threat that I would have expected from a similar situation (in fact, the protagonists almost seem like characters from a video game, essentially indestructible because the story must somehow move forward).

Having said this, I still found the film, despite my doubts about the story, well made and with a lot of care, pleasant and rather engaging especially in the last part which, although devoid of major twists (you will guess without difficulty which of the characters will be saved and what will they find at the end (if you have seen at least one zombie film in your life)  is nevertheless exciting and at times poetic. The young protagonist is very good, but she would have deserved a slightly more original story.