Mysterious and unearthly deaths start to occur while Professor Saxton is transporting the frozen remains of a primitive humanoid creature he found in Manchuria back to Europe.
The zombie/train combo has actually been done before, notably in 1972’s Horror Express, but Train To Busan is considerably more propulsive and less narratively cluttered.
It doesn't take a hardcore horror fan to know what's coming next, though such fans will have seen it before, notably in the 1972 Peter Cushing-Christopher Lee film Horror Express.