Train from Zagreb to Ljubljana

Took it the other way about ten years ago. Border control officers should board the train and check your passport on board. I don't think it is reserved. You might get cheap tickets a day before - 9 euros one way? Probably luggage racks above the seats.

https://www.slo-zeleznice.si/en/passenger-transport/travelling-abroad/croatia/zagreb
https://www.seat61.com/international-trains/trains-from-Croatia.htm

Thank you : )

Well, reserving is the only way to guarantee it, but it's probably not necessary

And on these trains, perhaps not even possible. (Could depend on which train it is.)

Thank you all so much!

I take the Zagreb -Ljubljana train every October on route to Germany. If you are going further into Austria or Germany you can pre-book on the Bahn.de website of the German rail network. Seniors qualify for big discounts. The border crossing passport check at the Slovenian border takes about 15 minutes and they come on the train - Croatia stamps your passport to exit, and Slovenia stamps your passport to enter the European Union Schengen Zone. No problems. In first class there is plenty of room for your luggage overhead in your compartment and you are often alone. No-one goes first class on this route any more, but I love it. I pre-book a window seat. Second class is full of students and locals, and looks pretty crowded. Other trains (not my early morning #112/212 express to Germany) may be a different story. Buying tickets a day or two in advance at the staton may be wise. I love this train route, misty mornings ...