I do like this very much for the girl's body language, her expression and the detail retained in the shadow over her eyes.
I don't feel comfortable with the image as-is but I'm hard pressed to explain why. If you don't mind I'll have a go:
Square format is very formal
, and the straight lines within this image, the window frame and the painted coach line, share this formality. The effect is reinforced by the black background that you have used in that the edges of the picture are sharply defined. The bottom edge of the window frame, however, is ever so slightly out of parallel with the edge of the picture; this is only significant in that it distracts from the essence of the composition - the contrast between the formality of the setting and the informality of the girls' posture.
That being said, I think you have captured the fleeting moment enviably well. I'd love it in my portfolio!