>>>?detective saves one girl by accident
Can’t he save her on purpose – or he saves her because the killer lets it happen? In this kind of story I feel like nothing should be accidental.

>>> eventually the killer gets too cocky and the detective finally captures him.
I want to see a supreme battle of wits and the detective bests the killer somehow but you feel like it could go either way. Getting caught because he was too cocky makes the ending anticlimactic because the antagonist failed rather than the protagonist won. I hope that makes sense. Think about the climax of Silence of the Lambs – Clarice is in pitch darkness and even then, the slightest noise (cocking the gun) caused by Buffalo Bill means she was able to shoot him. He had the upper hand and she still won… she’s that good. He didn’t just accidentally fall down his own hole – she bested him! That’s what the audience wants.

>>>The killer begins to allude to the detectives own memories of his dead wife and child, which sends him out of control.
Did the killer kill his wife and child or was he involved in any way? Or is he using that suggestion to manipulate the detective? Introduce this connection in the logline.

The detective being addicted to these dreams needs to be in the logline because, as dpg said, it’s his character flaw that can easily tie him to the rest of the story. Play up the idea of memories as drugs – that’s your hook. Make sure that is what people take away from your logline.

I don’t understand how the antagonist is going to steal their spotlight? As Nicholas pointed out, they currently have no spotlight. If his goal is to get fame then surely killing any old random group of people will get the attention he desires. Why these people? What makes them special? If they were famous actors, as Nicholas suggested, then I can understand why this would get more attention but I still see nothing that relates to the hero’s goal. Ideally, one hero taking charge, or whoever’s alive at the end (as in your other idea on the ship). As variable pointed out why not just contact the train authorities? Or get off the train? A contained setting is great but you need to explain why they can’t just get out of the situation that’s causing the problem.

This logline gives me more of an idea who the antagonist is but the heroes are blanks? You could always make the antagonist the protagonist? Could be a cool twist – we actually start wishing the actors dead and can’t wait to see how he kills the next one. Just gotta make him likeable. Or make the actors incredibly annoying…

What’s the incident that kicks the story off? Do they find a body? Does the train stop unexpectedly on a bridge with no means of escape?

Hope this helps.