I think Richiev’s suggestion addresses and fixes the things I would comment on, but it’s a bit long at 36 words.? So I think his version is a great framework to use to trim and rework for revisions.
Anyway, I’ll address all of the elements of the original logline specifically with a breakdown:
“A special unit fights against civilians that have turned into flesh-eating demons on a moving train that was hit by a strange meteor.” (23 words)

Inciting incident: “civilians that have turned into flesh-eating demons on a moving train that was hit by a strange meteor” —> For loglines the format we suggest is to put the inciting incident at the beginning. This is a bit long, and for the purposes of the logline,?how they change into demons isn’t necessary. The inciting incident is that they do.

Protagonist: “A special unit” —> As Richiev mentioned, consider describing one main character in the logline. Is there a character who goes through the most character development? Or one with the most to lose?

Goal: ” fights against civilians” —> This does not describe an objective. Think of the goal in terms of the climax; think of the climax as an?inevitable?outcome due to the inciting incident. If the inciting incident is that Brian Mills’ daughter is? kidnapped, the climax will be the the last confrontation between Mills and the kidnappers, the deciding battle of whether the daughter is rescued or not. So for your story:
Inciting incident: Train passengers turn into flesh-eating demons.
Climax: Protagonist faces demons(possibly the strongest, most distinguished of the demons, in other words, a main antagonist) and tries to defeat demons.

Antagonist: ” civilians that have turned into flesh-eating demons” —> Good.

Working off of Richiev’s version: a suggested edit: After a train’s passengers turn into flesh-eating demons, a federal agent must lead a ragtag group of survivors to the train’s engine so they can take control and escape. (29 words)

Overall, this concept sounds like a simple, solid idea for a horror film.