There are various ways you might attempt to access a train's fuel inventory from out of reach. Some of them work, and some don't, in inconsistent ways. It's not clear to me whether you're supposed to be able to do this at all.
- Not in map view, click on a locomotive: you can switch to the fuel tab (but can't shift-click fuel in).
- Not in map view, click on a cargo (or fluid) wagon: get "Cannot reach" message.
- In map view, click on a locomotive: works the same as not in map view.
- In map view, click on a cargo wagon: you can switch to the fuel tab, but only for one locomotive in the train. Which locomotive is chosen doesn't seem to depend on the cargo wagon clicked.
- Select the train from the train overview or a train station GUI: it picks one locomotive, and you can only access the fuel tab if you're within reach of that locomotive. The locomotive it chooses is not always the same as the one chosen when you click on a cargo wagon. (Example: have a train composed of a backward-facing locomotive, a forwards-facing locomotive, and then a cargo wagon. Put fuel in one of the locomotives to distinguish between them. Clicking on the cargo wagon from map view gives the fuel for the forwards-facing locomotive, but accessing the train from the train overview gives the backwards-facing locomotive.)
- Open the train GUI from within reach of the locomotive, and then move (or have the train move) out of reach. The fuel tab is darkened, and if you had it selected you switch back to the schedule. If you move back within reach, the tab is accessible again. If you open the GUI while out of reach, and then move in reach and back out of reach with it open, the fuel tab becomes inaccessible. For instance, if you open a train GUI and then that train drives right by you, the interface changes.