Obertraun to Milan by train

all possibilities at oebb.at

Unless you pick up the Vienna to Venice night train (via Salzburg/Villach/Urdine) at Atnang-Puchheim and then go from Venice to Milan, I would think that your travel would be over Brenner Pass (Innsbruck). In 2009, I went from Obertraun to Innsbruck via Stainach-Irdning and Zell am See, but I think it takes about the same time to go back to Salzburg via Atnang-Puchheim, then to Innsbruck by express train through Germany non-stop to Kufstein, then up the Inn to Innsbruck. I think the route via Stainach-Irdning is a bit more scenic than via Kufstein.