As Warren moves to check the other passengers for weapons, two things happen. First, the same kind of transparent blue overlay appears on four people in the train, red outlines highlighting four different handguns and two knives distributed between them. Secondly, a pair of shimmering blue crystals appear hovering near his hands, gleaming with a light that seemed to come from inside them, following the motions of his fingers in a way that intuitively makes sense to him. A glance up at the roof reveals more red highlights, concentric rings highlighting the location every time one of the creatures on the roof takes a step, a visual representation of the sound. He could probably aim at them like this, though not perfectly. As best he can tell, there are four of whatever the things are on the roof, and he has no idea if there are any more in the air.

Spoiler: Mechanics
Show
I'm gonna say you can throw ranged attacks with your crystals or one of the guns through the train roof with a 50% miss chance, with that perception check.