Hi!
I'm going to Ljubljana this September and I would like to take a night train from there to Budapest. I've been doing a lot of research but I can't seem to find anything. There's one, but it seems it runs only on July and August. Ideally I would like to depart from Ljubljana, but I'm also thinking on going to Zagreb and getting there the train (it's closer, so it seems more possible for a train to exist). I know this question's been asked before, but the schedules are constantly changing and all the answers from the posts I've seen were not reliable.
Thanks in advanced!

Train schedules in most European countries change once a year in mid-December. Unfortunately, there are no night trains in September, you'll have to either take a bus or day train, regardless of the city.

I am too late to help laia96 but perhaps I can help someone else reading this post. I used Euro Busways in June for a shared van ride from Zagreb to Budapest, taking me door-to-door in almost half the time it would have taken by train taking taxis into consideration. By train I would have needed to take a taxi from my hotel to the train station in Zagreb and then from the train station to my hotel in Budapest. It would have taken at least seven hours and I would have needed to deal with my luggage on the train. I was the only passenger in the van and the ride was four hours. The cost was 80 Euros, plus tip. www.eurobusways.com