You best Split this up (pun intended). Use ÖBB until Zagreb to reserve a seat on that Euronight, best check that soon as nighttrains are quite popular in summer.

Zagreb - Split cannot be reserved online as far as I know, the only option is to do that at the ticket office there afaik. But no panic, availability is usually good.

If you don’t like the uncertainty, ordinary tickets Bled-Zagreb and Zagreb-Split can be booked online at both +/-15 EUR; so cheaper than an the cost of an average pass day (possible 2 months before travelling).

Lesce Bled - Zagreb on the night train does not require seat reservations.

Zagreb - Split can be booked online for 1st July and later through tickets.oebb.at : add Interrail/Eurail as a discount and select one-way tickets. 3€

But as said regular tickets are quite cheap on this route.

Thank you @BrendanDB and @thibcabe.

I’ve checked the 15:19 Zagreb - Split train on ÖBB and it’s currently €53.40 + €3 reservation for a ticket. Is this more expensive than you were thinking? It’s still only June and I’m planning to make that journey on 30th August.

For the Lesce Bled - Zagreb part, the ÖBB site is still not showing tickets for the 07:16 Euronight direct, or the 07:23 Euronight to Ljubljana. Do you think it would be easy and cheap enough to buy a ticket from the station at Lesce Bled a couple of days before? There’s quite a lot of time to spend in Zagreb, so it's not like I will have to run between connections, but I’ve never Interrailed before and feeling a bit anxious!

Thank you!

Ben

A single ticket is around €15 on https://hzpp.hr/ including reservation.

Well if you have a Interrail/Eurail pass, you only need to pay the 3€ for the seat reservation through ÖBB.

For regular tickets it'll be cheaper to check the operator's website I assume.

The morning Lesce Bled - Zagreb trains cannot be booked. Add the train to the app like usual (so you get your ticket) and hop on. There will be loads of seats available, don't worry. The train empties in Ljubljana