Complex sentence: Moments later, the kid, back in his own seat, and wondering if he might have gone "a bit overboard there", hears a thump and sees a big smudge on the window from the woman throwing herself out of the train.
Split into simpler sentences: Moments later, the kid, back in his own seat, begins to wonder if he might have gone "a bit overboard". Right then, he is startled by a thump and and a big smudge on the train window; the woman had thrown herself out of the train.