A little more cropping to cut out the annoying structure in the upper right and to concentrate on the horses and the rail would have made this a much more interesting photo I think. Still a nice vote.
Wish we had a closer view of this. I'm sure it was probably zoom liimitation on the camera. Good action here though! I think, depending on how many Megapixels your camer is, you proabably could have cropped quite a bit, to bring them more into view. It looks like you could have waited til they were closer to you as well. I would have cropped quite a bit here...
Something odddly distracting about the red horse having all four feet off the ground :-) There's a lack of detail too - texture to the horses, and details of the flying dirt have been lost - perhaps too much zooming or cropping?
Great capture! If this had been adjusted in Photoshop with curves to make it just a little darker, it might have been a little clearer of the riders. And maybe a little more sharpening would have helped also. Just my thoughts, not being critical. It's a good picture overall.