We need more details. Which dates, which exact trains are you taking?

You can’t call an ambulance without stating the adress ;)

Its the train that leaves on 28th of july on 23:27 in Wien hfb Austria and arrives on the 29th of july on 11:25 in Rijeka Croatia 

Which train did you book precisely ?

- EN Wien Hbf - Salzburg Hbf 23:27 - 02:12

- NJ Salzburg Hbf - Villach Hbf 02:30 - 04:42

- EN Villach Hbf - Rijeka 06:25 - 11:25

By leaving at 23:27 the journey is quite risky and you'll need to use 2 pass days. I'd recommend leaving earlier, night trains can get heavily delayed.

There is only one carriage between Stuttgart, Munich and Rijeka which is attached to a regular train in Ljubljana. Unsurprisingly it is full on 28th July. 

Solution : there is a couchette available on 28th July between Munich/Salzburg and Ljubljana in a lady-only compartment.

If that doesn't apply to you, you could try to board a seating carriage as reservations don't seem to be mandatory from the Austrian border (per DB). However be prepared to sit/sleep on the floor…

In Ljubljana move to the regular train on which the couchette carriage is attached (departure 08:25).

Thank you for the information! I think i’ll sleep and extra night in wien and take a day train. 
 

Again thanks for the information! Have a nice day