Getting to Ludwig's Castles

It's feasible, but it would be rushed and a long day. It kind of depends on how early you want to leave Munich, how late you are willing to get to Salzburg, and how much time you think you need in Fuessen. For instance, there is a 6:51 train out of Munich. The connecting bus to Hohenschwangau would get you to the ticket kiosk at about 9 AM, an hour after it opens. It would be advantageous to have tour reservations. If you did, you should have time (5 hrs) to visit both castles and catch the 14:40 bus back to Fuessen Bahnhof where you would get a train to Salzburg via Munich (again). You'd get to Salzburg at 19:42. If you only wanted to tour Neuschwanstein, you could leave later, spend less time at Fuessen, and arrive in Salzburg earlier. Another option might be to leave Munich the night before and spend the night in Fuessen. That way you could easily get to the castles when they open, leave Fuessen sooner, and get to Salzburg earlier. I found these schedules on the German Rail website. Another, less hectic, trip might be to see one of Ludwig's other castles, the island palace of Herrenchiesee. That's directly on the way from Munich to Salzburg.

Going from Munich to Salzburg via Fuessen is kind of like going from N.Y. to D.C. by way of Pittsburgh. The fastest way from Fuessen to Salzburg goes back through Munich, so you are really doing a round trip to Fuessen followed by a trip from Munich to Salzburg. Even by regional train, the trip from Munich to Salzburg only takes 2 hours. If you did Fuessen as a day trip the day before, you could come back to Munich and wouldn't have to go to Salzburg until evening, giving you most of the day in Munich. For the single day trip (Munich-Fuessen-Munich-Salzburg), You'll have to leave too early to use a €29 Bayern-Ticket for the part to Fuessen. Full fare is €23 pP. Of course, if you go on a weekend day, you can use the Bayern-Ticket to Fuessen. Whichever, if you either go to Fuessen with a Bayern-Ticket, or get it in Fuessen after the castles, it will cover you from Fuessen to Salzburg. If you do Fuessen as a day trip, the next day, to make up for sightseeing missed the day before, you could leave Munich later and use the Bayern-Ticket. If you book early online and commit to a specific train, you can get a Sparpreis ticket (starting at €29) for a faster express train (EC or RJ). If you went to Fuessen the evening before, you could use a Bayern-Ticket, then get another one the next day for Fuessen to Salzburg after you see the castles.

Füssen isn't exactly on the way between Munich and Salzburg. Might make more sense to do it as a day trip from Munich - staying an extra night to do so - and then taking an early train direct to Salzburg the following morning.

Look at a map and determine whether you want to do that. It might be possible, but will make a long day.

"it may be doable by train" Of course it is. Most trains (all regional ones) between Munich and Salzburg stop at Prien a Chiemsee Bahnhof. From there it is a short walk or a streetcar ride to the harbor and the boat to the island. There are lockers on the platform at Prien Bahnhof if you don't want to schlep your luggage. "Linderhof ... is really only accessible by car" Oh? I did it without a car. I was staying in Oberammergau, which I reached without a car - by train from Munich. That's where I started my Romantic Road trip, which I also did without needing a car. I caught a bus from Oberammergau Post/Bahnhof, 25 min to Linderhof. "going by train for two is like renting the train train twice while you only rent the car once" Interesting comparison except renting a car once costs far more than train tickets for two. A Bayern-Ticket Single from Munich to Salzburg costs €21. Just fuel for a compact car will cost almost €20. If you don't already have an Austria road sticker, that's another €7,70. And we haven't even addressed the cost of a day's car rental. A second person on the Bayern-Ticket is only €8 (about 1/3) more. Three more people costs the same, €29. By the time you get to 5 people, you will probably need a bigger, more expensive car.

The only one of Ludwig's three castles that is between Salzburg and Munich is Herrenchiemsee. It is a palace modeled after Versailles that sits on an island in the Chiemsee which is right on the A8 autobahn from Munich to Salzburg. I have only been there by car but it is truly a beautiful place to see and it may be doable by train or bus and is certainly easier than going to Fussen. My German cousins have always dismissed Neuschwanstein as theater anyhow. The other one is Linderhof which is a true work of art but is really only accessible by car although is is the jewel of the set IMHO.
How about renting a car for one day? "We" indicates that there will be at least 2 of you going. Remember going by train for two is like renting the train train twice while you only rent the car once.

ViaMichelin says the vignette is needed and costs €7,70, but might have out-of-date price information. One way to avoid the vignette is to park in Freilassing, just across the river in Germany, and take the S-Bahn (€2,10 pP) into Salzburg Hbf. I think there is a bus from downtown Freilassing (Rathaus) that gets you closer to the Altstadt.

10 day Austrian vignette is 7.90 Euro as of last week when I bought one at ADAC.