I always used to think he was shot from behind, but after some discussions on here and some rethinking I'm pretty sure the crash theory is the one to go with. The conductor's chair is very close to the window, so it would have been possibly to crash into the glass and back into the seat. Also, the force produced by a body being flung with 30 or so mph into a solid object is tremendous and could well crash a head open.
I also have to disagree on the photo showing Chuck. I don't think there is a lot of resemblance and it is not taken away after Chuck appears either.
The pic disappears after Duck has died, so my guess would be that Kenny takes it off.