Four of us will be travelling from Zagreb to Ljubljana and also from Ljubljana to Trieste, the second leg being on a Sunday.
Can anyone help by giving me suggestions and the best site to book with please?

For Zagreb to Ljubljana see:

https://www.seat61.com/international...greb-Ljubljana

For Ljubljana to Trieste I took a bus. Sunday is often a bad day to travel in Europe - reduced schedules and maintenance work.

When is your trip? We will be traveling by train from Zagreb to Ljubljana in early May. According to the Man in Seat 61, it looks like we cannot make train reservations from Zagreb to Ljubljana online. We have to go to the train station to make our reservation and purchase the ticket. If I am wrong, I hope someone can correct me. Thanks.

Well, he says this:" You can easily buy tickets from Zagreb to Ljubljana or vice versa at the station for around €18, or for just €9 with an advance-purchase ticket if you buy the day before or earlier. They cannot be bought online and there's no need to buy way ahead as there are always places available. "

What is your problem with buying at the station? It's not even clear you can buy a reservation at all, just a ticket.

Some folks just want everything buttoned down before going - understandable - bu in this case thursdayds and Seat61 no doubt tell you buying at station is easy - if there was a way to buy online www.seat61.com would say it. Try googling the national railway of the countries and get official site and see - not sure this site books tickets but they do in many European countries at same prices: www.trainline.eu. Can think of no other sources to help out.

Thank you. My wife just informed me that I was wrong as we are travelling to Trieste on a Monday. The trip from Zagreb to Ljubljana is the one on a Sunday. Any particular reason that you took the bus to Trieste?

You have been very helpful.

At the time there was no direct train, but I see on seat61.com that that has changed. So now a train would be preferable. I hope you are spending more than one night in Ljubljana?

PalenQ, you are correct that "some folks just want everything buttoned down before going". And I am definitely one of those folks. I like to have all reservations made, if possible, before I leave home. I'm just surprised, and maybe a little ignorant, that one could purchase train tickets for the day and time they want on the day of. I did try www.bahn.de and cannot purchase/reserve tickets there, either. I assume one can buy tickets upon arrival in Zagreb, a few days before departure to Ljubljana.

Rogandgee, thank you for posting this timely thread. I was planning on posting a similar thread, but you beat me to it.

Trains aren't like planes. Worst case, you stand, but aside from a few special cases like some night trains or high speed trains (and maybe Eurostar?) you won't be denied boarding. It is better (i.e. cheaper) to buy tickets ahead of time for long distance trains in western Europe, but even there I wouldn't bother to book ahead for shorter distances. I was just in the UK, and while I booked my long distance trains ahead of time, I bought other tickets the day before or day of. If there may be a language problem I write down what I want and hand the paper to the clerk. E.g. Zagreb - Ljubljana - date - time - class - number. Include the train number if you know it.

I don't think the trains between Zagreb and Ljubljana are reserved. The one I took nine years ago wasn't anyway, and I don't think that's changed. Perhaps certain trains are. I'd just get to the station a day before and buy tickets to get the cheaper fare. Zagreb's train station is easy to get to - there is a tram stop right in front of it.

www.bahn.de/en only sells tickets on trains involving Germany. Again maybe www.trainline.eu would?

Thank you everyone for all your useful information. My wife and I have travelled by train elsewhere in Europe and concluded that, other then for the very long distance routes, we should simply buy the tickets on the day of travel giving us absolute flexibility. The reason that I started this tread was because I had no idea what prevailed in that part of Europe as we have commitments in Trieste and do not want to be late getting there.
Once again, thanks. I love this method of sharing ideas and gathering information.

PalenQ, Thank you for the links. I checked trainline.eu, and they only show the bus schedules (GoOpti and Flixbus) for Zagreb to Ljubljana. So it looks like our only option is to purchase the tickets at the train station, either the day of or a few days before. There is a 12:36 pm departure, which works for us. I don't have a lot of experience traveling on trains in Europe. We did take the train from Prague to Krakow in 2003, but I think my daughter who was studying in Prague at the time purchased the tickets for us. Then last year when we traveled by train through Andalucia we purchased our tickets online when they went on sale to get the lower price. So it looks like central/eastern Europe is different than western Europe. I appreciate your assistance. Thank you!

It would seem that that's just what thursdaysd (who IME, only posts when she has checked current information) said upthread.

I've been to Split Train Station several times, though not to catch a train. I get the impression, but might be wrong, that rail travel isn't one of the favoured modes of travel in Croatia.

Perhaps because only a few cities in Croatia are served by trains?

I'm a train nut but those buses most say are pretty comfy too - I'd take whatever's quicker and cheaper. RailEurope may book those trains - but usually at a higher price but if really want reservations and tickets check them out - call sites I list far above for good personal service.

I've taken trains only between Ljubljana-Zagreb and Ljubljana-Rijeka. These were direct trains and worked great for me, but I'm not sure if they were Slovenian or Croatian trains. Slovenia doesn't have a large rail network, either.

Lots of towns are served by trains in Croatia. But Croatia, like some other eastern European countries and unlike western European countries, hasn't invested to maintain its rail network, and what remains now is limited largely to local trains. You can train between Split and Zagreb. You can train between Rijeka and Zagreb. But you can't train between Split and Dubrovnik. (Dubrovnik had rail service until 1976.) Croatia has invested in its highway network instead.

Here's a map of the train lines in Croatia. I leave it to others to decide what counts as a city or a town.
https://www.croatiatraveller.com/Transport/TrainMap.htm

In summer I think the ferries that go along the coast are running - not cruise ships but something in between?