Well, last direct EuroCity train from Verona (most of trains to Austria/Germany originate from here) to Innsbruck leaves 17:02 (arr. 20:32), thus in order to catch you'll have to depart from Milan around 15. If it's not possible then you'll have to travel with two changes: Milan Centrale-Verona Porta Nuova (18:05-19:27), then Verona Porta Nuova-Brenner (19:50-22:52) and finally Brenner-Innsburck (23:04-23:44)
On TrenItalia website you can purchase even right now ticket Milan-Brenner, however all the discounted tickets for this connection (for June 15) already sold out, so only standard full fare tickets are left (=42,5 euro). Ticket Brenner-Innsbruck is 7,8 euro when purchase in Brenner (or via
Austrian Railways website). On the contrary, if you ok leaving Milan earlier, then you can buy Verona-Innbruck discounted ticket for the last direct EuroCity train (17:02-20:32) for 29 euro, while Milan-Verona (for instance 15:05-16:27) will cost you either 19 euro (discounted purchased in advance) or 21,5 euro (standard full fare)
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