Dark Metro is a horror manga placed in a metro, where spirits with grudges haunt people. The story isn't bad, a haunted metro and people falling victims to the ghosts, while Seiya does his best to save them. The problem with the story is (besides generic horror plot) that there isn't much of it at all, it's a clump of couple of people's fates and the timeline moves too fast, all you can see is action that characters are overreacting, screaming or running for their lives, nothing more.
The art is okay, there aren't many details but metro and gore part are done very well.
There are too many characters, each one but Seiya didn't get much time at all and all we could see was very short backstory, while the rest was running away from the ghosts, Seiya comes, does his thing and the story moves to the next characters. Despite that, most of the characters were overreacting all the time, behaving violently and bringing others into danger, which made them much more annoying. Seiya is the only character who is in all the chapters, though he seems more like "deus ex machina" than a real person, there is barely any his character development because he mostly appears in time of danger and disappears afterwards.
Even though this is mediocre manga at it's best, it's not bad and has enjoyable parts. Many elements are generic - flow of people who get into trouble, an all-mighty protagonist who saves others from evil, haunted place and ghosts.