Questions & Answers about Я не знаю, куда идёт поезд.
Why use куда instead of где?
куда asks “to where” (direction or destination). где asks “where” in the sense of a static location. In our sentence the train is moving toward some place, so we need куда.
Why isn’t there a conjunction like что or the particle ли after не знаю?
In Russian indirect questions introduced by a question word (куда, когда, почему etc.) you drop что/ли and go straight to the question word. So Я не знаю куда идёт поезд is correct (with a comma before куда, see below).
Why is the verb идёт used for a train? Shouldn’t it be едет?
Both идти and ехать mean “to go,” but ехать is for wheeled transport in motion, and идти literally means “to go on foot.” In timetables or announcements Russians traditionally say поезд идёт (the train “goes”) even though it moves on rails. Informally you might also hear поезд едет, but идёт is very common in this context.
What case is поезд, and why isn’t there a preposition?