The Camelback is the correct choice, but the routing was not correct to make 16 stops, and you need to maximize the engines speed. I can see from your screenshot you did not do this. First, the route:
02. Rock Springs
04. Twin Falls
16. Great Falls
Forget about the offer early on from Missoula about the firebox. Don't fall for it.
To max the train speed (this will increase the chances of a breakdown so I recommend saving each day when you haven't had a breakdown) go to the consist/routing screen at the very beginning of the scenario. Above and to the right of the water level indicater is the throttle. You need to click on it, maybe several times and put the needle to the bottom of the red instead of where it is just at the bottom of the black. If you do end up having a breakdown just reload your last save and try again from there.