Professionals don't leave with half full tanks. Ask any airplane pilot if they truly trust their gauges.
What is your basis for this statement? Are you, or have you ever been, a professional locomotive engineer?
I am, and I can tell you I have departed on trips with locomotives with FAR less than half a tank of fuel.
As for the gauges comment, you're right, gauges do break. The design of the Genesis locomotive already compensated for that. Along with having fuel redputs in the computer screen in the cab, and digital readouts next to each fuel filler, there are fuel sight glasses on both sides of the locomotive so Full, 3/4, 1/2, and nearly Empty can be seen at all times. It's physically impossible for the sight glasses to be wrong.
We don't know the back story behind this. It's entirely possible the fueling station in Chicago was broken, and the train was sent out anyway to avoid being late. It's also possible that the train is normally fueled in Carbondale rather than Chicago. This is done all the time in the railroad industry. The railroads fuel wherever diesel is cheaper. If Amtrak can get cheaper diesel in Carbondale than Chicago, then they may always fuel down there. Lastly, it's possible that a fueled locomotive which was on the train had a problem and they needed to change equipment. In that case they may have just put a locomotive on with only enough fuel to make it to Carbondale.
It's also feasible that this wasn't even Amtrak's fault. What's to say the fuel company wasn't late bringing the fuel truck? That's entirely possible given the weather.
It's an unfortunate situation, but a late departure is a lot better than running out of fuel in the middle of nowhere.
I don't appreciate calling me and my brothers and sisters "unprofessional" because of it. My job is to operate the train in a safe and efficient manner. As a locomotive engineer at Metra I'm given a number for the amount of fuel required to make the trip, and another number they want me to have as spare fuel to account for delays. I simply verify I have enough fuel to meet those requirements. I have no control over how the railroad chooses to fill my equipment. If I don't have enough fuel to make my trip(s), I notify someone. That's it. I don't order the fuel, and if for some reason I'm told to just run with what I have that's exactly what I have to do.
EDIT-Here's a photo showing both the digital fuel display and the sight glasses.