Zagreb to Trieste

I don't remember a post about that specific trip, but there have been multiple positive comments about GoOpti on the forum.

I believe any mode of transportation will route you through Ljubljana, which is a lovely city very much worth 2 nights (or more).

I would agree with acraven. When routed through Ljubljana it’s well worth a stop for a few nights. GoOpti (I recommended their transfer in your other posting). We spent 4 nights there and it ended up being one of our favorite cities anywhere! We stayed at Meščanka on the River. Wonderful apartment!

I should have mentioned that Zagreb and Ljubljana are linked by both trains and buses. I think the buses are probably somewhat more frequent. That part of the trip could be made via public transportation, so you could just pay for a shuttle on the Ljubljana-Trieste leg.

That makes sense, acraven. We used GoOpti from Ljubljana to Trieste and it worked well. However, from Budapest to Ljubljana we used MyDayTrip.com (private transfer) which was even better (and, of course, more expensive).

EDITED to add: I should have stated that our GoOpti transfer was a multi-passenger van. In the pouring rain.

We took the train from Zagreb to Ljubljana, and it was a very pleasant and very easy ride. It's about 2.5 hours and goes through pretty countryside following a river part of the way. It was very inexpensive in September 2019; I think about 9 euros one way. The Zagreb train station is small and easy to navigate.

There are trains between Ljubljana and Trieste as well. They are not fast, but it is a pretty scenic route. https://www.seat61.com/trains-and-routes/venice-to-ljubljana-and-zagreb-by-train.htm

I used GoOpti in the middle of the night in Ljubljana (airport run). Very easy to use and reliable.

There are also trains (at least in non-COVID times) running from Trieste-->Ljubljana-->Zagreb

I believe a bus would be faster than the train for the full Zagreb-Trieste trip. The Ljubljana-Trieste section seems like a bit of a slog by train. Although trains are typically faster than buses in western Europe, the situation can be quite different in the Balkans.

Or, if you wish, you might just take the train part way, then get off and jump in the back of a truck filled with flowers, after hitting the driver and tying him up. You could then travel until daylight, avoiding helicopters flown by men in dark clothes and caps, finally meandering to the coast, stealing a motor boat and crossing the northern part of the Adriatic and tossing fuel drums into the water and igniting them with a flare gun. If you veer right, you could end up in Trieste instead of Venice. It worked for James Bond so it's probably doable.

Thanks so much for the info. I am hopeful that we will be able to travel. Think we will try Go Opti.

I use the train to Ljubjana. You can pre book via Bahn.de and get a first class compartment to yourself if you take the early train. If you are over a certain age there is a big discount.
Whatever you choose, be aware that Slovenia is the outer EU border, and they were being very strict in earlier Covid months. Check any rules about testing. Not like it used to be pre pandemic.