On the other hand, if I "touch" the train in some ways, it will "fix" itself. See this animation of a train with this problem: observe the following:
- I have a cargo wagon with coal,
- I have only one inserter arm unloading the wagon into a chest at the offending position,
- This insert is not working
- I place a third wagon,
- The second wagon magically starts to be unloaded again.
I could not find a reliable bullet proof repro -- it appears to be something that happens "sometimes." Because this event happening is somewhat unpredictable, please see this link to a save game file. The player character is near the two trains exhibiting this behavior (the coal and stone train). The only thing unusual about these trains I can see is that it has only one stop (the "unload" station), since the resource patch it was transporting material for had the resource exhausted.