Thats far, way too much-even more as a normal ticket should cost. perhaps confused with another town named Trento? There are several of them in ITaly. I guess you mean the one south of Bolzano and north of Verona.

The only=rather cumbersome and quite slow way to do it without any extra to pay is using local trains, and not the 2 hourly through trains running München-Kufstein-Innsbruck-Brennero-Bolzano-Trento-and on to Milano/Venezia/Firenze. However, using these trains (by OeBB) will cost you an extra 10€ as supplement (can include for free a seat REServation if done before at a OeBB counter-or pay on train to conductor without any garanteed seat) and thats all.

There is however even a snag at the IT side-the local trains south of Brennero are now not by TrenItalia anymore but the local regional SAD (Dolomiti) and its unclear if they accept these passes-or not.

Note-if you want to travel further into Italy, then ALL IC and even more the superfast FRecce (=arrow) trains must be reserved and this has to be done manually at ticket counters. The REServation for IC is only 3 or 4€, for the FR much higher and varies with distance and time of day/weekday. The R-egional treni TrenItalia are free, just hop on. DO also look carefully into the much more strict covid rules that IT has! There are however several other regions who were so grossly unsatisfied with the level of ´servizio´ from TrIt that they took it in their own hands and started their very own local railway-its unclear how passes are accepted on them (f.e. TPER in Emilia Romagna)

Where did you look? A reservation (optional) should be 3 EUR via the ÖBB website and there's the 10 EUR Interrail supplement for this route.

Funny, now there is a possiblity to go without costs. There was no option at first. Thank you. 

You can close this 

Please note, that on the Regional Train that runs directly from Innsbruck to Trento Interrail is maybe not valid on the italian part (we are not sure if SAD accepts Interrail or not). If you choose one of the trains with changing at Brennero/Brenner you can go without paying (Trains are operated by Trenitalia on the italian part). 

Like others say, for the Eurocity you pay (also on board, ticket offices or online at tickets.oebb.at) 10€ supplement for the italian part and 3€ for the optional reservation via ÖBB.