Just in case, since due to limited number of tickets the discounted fare for Zagreb-Zurich isn't guaranteed, it is also good to know that the full fare for ticket Zagreb-Zurich is around 115 EUR.
Also, if you are ok partly substituting train for other mean of transport and avoiding Zagreb, then there is totally different options available as well. You can travel Split-Rijeka by bus or ferry, afterward there are two choices:
a) from Rijeka continue by train to Ljubljana where you can catch the same overnight train #414 to Zurich (dep.20:49). Full fare is 98.6 EUR, but also same SparSchiene tickets are available (seats from 29 EUR, couchette from 39 EUR, 3 bed sleeper - 59 EUR)
b) from Rijeka continue by bus to Italy (Trieste) -
http://www.saf.ud.it/cms/data/pages/files/000057_resource1_orig.pdf. From there you can easily catch daytime trains to Zurich (usually with a change in Milan taking app. 8-9 hours in total). TrenItalia website also offers discounted tickets for certain connections and those can be bought online (
www.trenitalia.com , need to choose tariff "Smart")