I tried painting my own version and pretty happy with it. It's definitely not as awesome, but I think the experience of copying is very interesting. I didn't get the horse quite right. In awe of how he captured it's form.
I drew lines over the original to study composition. I noticed how the bottom half of the horse's ass is in shadow and fits under a diagonal intersect and the lighter half of the torso is above the diagonal. Also, the horizon is pretty much two thirds up the paper following the "rule of thirds." Even the smoke from the train follows a diagonal in the NW quadrant. It's very interesting.