Train question

You'd do best to take the Mittenwald Railway to Innsbruck. It's so much closer, and you're right about the scenery being breathtaking.

But it could be a lot more expensive going through Austria. Through Munich you can use the Bayern ticket costing 46 EUR for 4 people and taking 3 1/2 hours. Through Innsbruck it would cost at least 76 EUR and you would need to buy a nonrefundable Sparschiene ticket now to get that price.

You'd do best to take the Mittenwald Railway to Innsbruck. It's so much closer

That's a common mistake. Actually, both routes are almost equal (remember that there is no direct train line between Innsbruck and Salzburg; the train runs either via Rosenheim in Bavaria or via Schwarzach-St. Veit, circumnavigating the Wilder Kaiser mountains either in the west or in the east). And you can book those cheap Sparschiene tickets only after the new winter time tables are published (mid of October), so it's not exactly clear how much they will cost. Combining local train tickets for the German leg with an Austrian "Einfach raus" ticket would cost exactly twice as much as a Bayern ticket. But yes, the route via Innsbruck is more scenic, and if I had time I'd prefer the route via Schwarzach and Bischofshofen any time.

At the moment you may buy online tickets from OeBB until December 14th, 2019.
On the following weekend there is the yearly European coordinated time table change of all railway companies. All train information past this date is still tentative, but major train connections normally do not change.

I looked up the prices for December 12th, 2019 for going from Garmisch-Partenkirchen to Salzburg:
The "Sparschiene" bargain ticket is €19 when going via Innsbruck, continuing on either route to Salzburg, i.e. direct or via Schwarzach-St. Veit. (The price is for one person, 2nd class; ticket bound to selected train.)

Thanks, all of you! I always get good train answers on the forum. Scenery is my priority! So Mittenwald to Innsbruck, then via Schwazrach and Bischofshofen to Salzburg will be the plan. Even at double the price, the ticket divided by four friends still sounds like a deal. So now I wait for the winter time table to be published in October. I'm sure I'll be back with more questions then...probably about the "Einfach raus" ticket. Thanks again!

wmt1, I was typing while you were posting. Do I need to buy that ticket ahead of time to get the 19 euro fare? Also, for the "via" I put in Innsbruck, but is there a way to add "via Schwarzach"? Your reply was very helpful! Thanks!

Also, for the "via" I put in Innsbruck, but is there a way to add "via Schwarzach"?

Just insert "Schwarzach-St. Veith" instead of Innsbruck. Travel time will be 5:46, but it's definitely worth it.
The train will first descend from Mittenwald to the Inn valley (take your seat on the right side for splendid views), then it follows the Inn Valley until Wörgl. Then it will climb up the Brixental in serpentines to Kitzbühel. From here you should take your seat to the left to see (from east to west) the Wilder Kaiser Mountains, the Loferer Steinberge and the Steinerne Meer ("Stone Sea"). From Zell am See the train follows the Salzach river, which in some places narrows into a gorge. The faster route through Bavaria is also nice but does not have the same dramatic high alpine but lovely pre-alpine scenery.

You can add a second "via" to enter "Schwarzach-St. Veit" (this is the correct spelling).

Regarding the "Sparschiene " ticket it is similar to airline tickets. There is only a certain number of tickets available, and the price may rise depending on the overall booking situation and the days left until departure. The method how this is calculated is one of the best kept secrets of OeBB.

The number of train changes depends on the time of day, two is the minimum.

The price given was for one person, including the section in Germany (no need for a Bayern ticket).

If you want to further crank down the costs, using group tickets, i.e. Bayern ticket in Germany and Einfach raus in Austria, you are not allowed to use high-speed trains (e.g. RailJet). You must take R, REX or S trains (regional, regional express or Schnellbahn [fast commuter] respectively).