There’s a lot to unwrap here…you’re giving us too much information, and tossing it out like dollar bills at a cheap strip club. Rather than make the story sound compelling, this merely raises questions just to understand the thing: If he’s a secret agent, what’s he doing with a criminal? Is he undercover? If so, who’s the criminal? A partner? An informant? If so, his status as agent ain’t much of a secret. How much security does a train have? Are they trained to handle events of this nature? If there’s some type of emergency, wouldn’t they just stop the train? Why don’t they stop the train? Why are any of these people on the train? How does a meteor possess these people? What does any of this have to do with our hero the not-so-secret agent and his apparently compliant criminal?

Maybe make the agent something like a U.S. Marshall so he has a reason to be transporting a criminal, and instead of working together the bad guy takes advantage of the distraction to try and escape, so our hero not only has to recapture & contain the guy but contend with all this other crazy shit going on. You can even really stack the deck against him by eliminating and/or infecting security, so he’s the only one who can save the day. And instead of the train being hit by a meteorite (proper name once it impacts Earth) maybe the train passes by an impact site and passengers are affected by the radiation or something…that way the train can keep moving, and the infection now threatens all the upcoming destinations.

As for the logline, stick with the basics: protagonist, antagonist, conflict, stakes.? “A U.S. Marshall transporting a criminal by train must contend with violently possessed passengers when they ride through a mysterious meteorite’s impact zone.” Not very graceful, but one can always play around with the phrasing to make it flow better, and the important thing is that?the story & characters are clear. If they’re not clear in the logline, odds are they won’t be clear in the script.