Getting from Munich to Garmisch-Partenkirchen

There is construction work on the rail line during that time. You can still get to G-P, but you need to take the rail replacement bus to Weilheim, then board the continuation of the train to G-P.

Trains appear to be running normally of June 17.

thank you! I did do another search and found a return train. Getting there will just be a little less smooth than i had hoped due to the construction. I hope maybe my hotel concierge can walk me through the logistics.

The replacement buses are indicated at Munich Hbf Nord, north side of the station, Munich Hbf is a giant construction project on its own right now, so I would want to have the bus pick-up location identified beforehand. Depending on where you are staying in Munich, it might be easier to get the bus at Donnersbergerbruecke S-Bahn station where it also stops.

Flixbus luggage sizes:

https://global.flixbus.com/service/luggage

I don't know where you intend to visit from GaPa. You only have 3 full days and there's plenty to see and do around there using the free public transportation pass you'll receieve where you are staying.

Fussen and the two castles, if in your plans, is quicker to reach by car.

Could the trains be sold out?

No. Trains are mass transit. If they sold out they would be useless. Most people that travel on those trains have decided to make that trip, and bought their tickets on the day themselves.

Bus substitutions are annoying, but they work, and are usually well organised. See it as a relay race with you as the baton. So you get out of one vehicle and follow the crowd to the next vehicle.

There is a map here that shows where the SEV bus will leave from.
SEV stands for SchienenErsatzVerkerhr, Rail Replacement Bus in English. Pay attention to signs with that mentioned on it.

https://www.bahnhof.de/downloads/replacement-service-maps/4234.pdf

So the SEV stop is in Elisenstrasse, as your train goes direction Pasing, one block away to the North.

make the decision on whether you really need the car around Garmisch. I would say no but if you think yes then you may as well get it in Munich. Do not use airport or desks actually in railway stations, they charge higher rates. Sixt have an office close to the Ostbahnhof (the branch name is München Haidhausen). https://www.sixt.de/mietwagen/deutschland/muenchen/muenchen-haidhausen/

If you do have a car I would recommend staying more rural than in Garmisch, try https://klosterhotel-ettal.de/en/ to the north, very well placed for Linderhof Palace (forget going to Fussen) and Oberamergau.

And if you decide on train but aren't definate about Garmisch (which is a bit of a sprawl) try Murnau which is a shorter journey from Munich. https://www.tourismus.murnau.de/

"I was planning to go to Garmisch (June 13),"

Is it necessary to sleep in Munich on the night of June 12? Have a look at the DB schedule for train options on June 12. I would choose to leave Munich late that same afternoon, after I was finished with all my sightseeing, and arrive in G-P that same evening - when trains can get you there according to the DB schedule. The S6 (S-Bahn Train) from Munich Hbf is scheduled to take passengers to Tutzing, where they board a connecting train to G-P. Travel time is roughly 15 minutes less than by bus. This routing appears all day long until around 23:00, when the replacement buses kick in.

I experienced the same train delay for my family this past week and decided to hire a driver with Day Trip. It is expensive but I arranged for a side trip to Linderhof on the way to Garmisch. It was a safe and excellent trip.