Zadar has trains to Zagreb but the route has fallen into disuse. There are at least a dozen stops along the way making it a very, very long trip. The best way of getting from Zagreb to Zadar during the day is to take a bus.

Occasionally it is convenient to take a night train and there you're in luck as there is a nightly train with sleeping accommodations. Taking a night train makes sense for saving on accommodation and also if you need to transport a bike.

Otherwise, take a bus or fly.

Trains to Croatia