"Di mana kereta apinya?"
Terjemahan:Where is the train?
Where is THE train? = tunggal.
"Train atau trains?"
Sederhana. Jika ada satu kereta, itu "train", jika ada lebih dari satu kereta, itu "trains".
I don't understand what exactly the point you don't understand. What are the other question with "trains"?
I have to take the train, where is the train ?= a train, my train = tunggal.
In my town, there are several trains. = jamak.