After talking to Soapy, run to front of train towards the water. Leave your horse behind the rock near the bridge. Run back to the engine of the train and stay in front. Wait for the two riders to come through. Use your quick draw to take them out. Take their horses and leave them with your horse. Take cover in front of the engine again. Two more riders will appear. You can see them riding quickly around the train on your map. Use the same method. Then, go to one of the horses and ride him around the rock towards the cannon. Use quick draw to take out the two men on either side of Dutchie. Your horse will probably die and you will have to take a shot or perhaps two, but keep moving towards the back of the train and into tunnel until you find cover behind it. Sneak around to the right on the same side you started on. Kill the man on top of the car first, then the man standing by the rail on the same car. Next, squeeze between the two cars and aim directly for the end of the cannon with whatever weapon desired. Shoot Dutchie until he is dead.