My first Korean zombie movie, I'm pretty sure. A free-to-me streamer on Amazon Prime.
Seok-woo is a semi-big shot fund manager in (I think) Seoul, dealing with some unexpected market turbulence. You can see he's concerned, but he should be more concerned with the underlying cause. Which is a zombie outbreak. But he's clueless. And also a clueless dad to cute-as-a-button Soo-an. He misses her recital. He buys her a Wii, but he doesn't know that she already has a Wii. And also a clueless husband, because his ex-wife lives in … guess where? Yup, Busan.
So they're off on the titular train, but it soon becomes painfully obvious that Korean civilization is quickly collapsing around them. And worse, a zombie made it onto the train, setting off a wave of infection among the crew and passengers, with accompanying chaos.
It's pretty good, albeit predictable. In the sense that "you've seen one zombie movie you've seen them all": there's not going to be many of the characters left at the end. There are some class warfare zingers: Seok-woo's fund is (somehow) involved with the corporate misfeasance that kicks everything off; and the one really bad guy is a train passenger in a business suit who's more than willing to sacrifice other passengers to survive.