
When adding another wagon at the end the inserter works just fine and inserts the items in the wagon, but without extra wagon the inserter just doesn't realize there's a train even though it could insert.
To reproduce: have a train with 7 wagons at a station, build all inserters that can load into the 6th wagon, then take 7th wagon away, and the next time train enters the station the last inserter won't load the 6th wagon.