"The train is going."
Translation:Die Bahn fährt.
It's fine (there was just a bug that needed fixing).
Die Bahn can have a broader meaning, often meaning "the railway system" but also colloquially understood as "the train" in the right context. It can also kind of mean "track" in other contexts (usually specified as part of a compound word, but shortened to simply Bahn when the context is clear): A bowling alley is a (Kegel)Bahn, the orbit of a planet is a (Umlauf)Bahn, and you might have heard of the Autobahn.
Der Zug is just the vehicle travelling along the tracks (engine and carriages together).
I still do not understand why Zug is incorrect. According to the explanation below Der Zug is a vehicle traveling along the track, which is what the English sentence is saying "The train is going" not "The railway system is going". I entered Der Zug geht and it was marked wrong....why?
Replying to publicum, AZ_p and Stephen83116: Having initially been marked incorrect for "Der Zug geht", and learning that fahren was the verb to use, I then tried "Der Zug fährt" and can confirm that Duolingo accepted this as correct. :-)
P.S. CapnDoug said as much 4 months ago but I hadn't spotted that post!