Montreaux to Fussen Overnight?

No, you do not want to overnight from Montreux to Füssen because there are no direct trains; you must transfer a minimum of three times. The same with Munich although I see two changes required for this market.

you need to understand that Füssen is a small town at the end of a minor branch line.

If it could be done reasonably you would need a sleeper train between two major cities and then a branch line train.

All trains to Füssen originate in Munich.

So if, for example, you wanted to start at 7pm in order to get to Munich by 6am.
Montreux dep 19:11 IR 1831 Interregio Direction: Brig

Visp arr 20:23

Travel time 1h12m

Transfer time 5 min.

Visp dep 20:28 EC 56 Eurocity Direction: Basel SBB

Basel SBB arr 22:32

Travel time 2h4m

Transfer time 41 min.

Basel SBB dep 23:13 IC 60402 Intercity Direction: Amsterdam Centraal

Mannheim Hbf arr 01:49

Travel time 2h36m

Transfer time 15 min.

Mannheim Hbf dep 02:04 ICE 619 Intercity-Express Direction: München Hbf

Augsburg Hbf arr 05:32

Travel time 3h28m

Finally on the final leg a longer segment. Only problem, none of these none of these is a sleeper train so they will be stopping all the time for stations. For example, this last one which you boarded at 2 in the morning (which only is scheduled to stop for 2 minutes) also calls at Heidelberg at 02:19, Stuttgart at 03:40, Ulm at 04:42, Günzburg at 04:58, before arriving at Augsburg at 05:32.

Then you need a train to Füssen. The first one is at 04:50 but you will not catch that unless you leave Montreux a full hour earlier (same pattern) but there's one at 6:21 which is direct and gets to Füssen at 8:37, just in time to start your day. It does stop 16 times on its 2:16 minute journey on the way so you'll have plenty of time to look around.

Big advice - none of the trains you will be on except the one to Basel and the last into Füssen end their journey where you change. For example, as you see above, the train to Mannheim stops for 2 minutes and then continues to Amsterdam. If you miss your stop, you won't be happy.

Montreux to Füssen is not a trip that makes sense at night. It is really too far and too complicated for a daytime journey too. Can you arrange a stopover on the way? Maybe Basel? Maybe Augsburg? Both beautiful and steeped in loads of history. Basel, one highlight is the chain ferry. Many others. Augsburg - two of the many highlights are the church that Martin Luther made major pronouncements at, and the Fuggerei (complete with bomb shelter open to the public).

Same route pretty much Munich to Interlaken. Munich is the end of the train from Mannheim to Augsburg, and when you reach Basel change for a train to Interlaken West and Interlaken Ost.

Thank you 🙏 I wonder then if it would be easier to rent a car and drive this? It seems everywhere we have considered to see castles and WW2 history take us very far out of our way and eat up too much of our ten day trip which starts and ends in Geneva.

Nigel has given you a detailed and well explained analysis of your plan . I don't want to be negative , but consider - Trains are invariably faster than cars , with far less stress ( no concentrating on the road , traffic , etc . ) As it is , the train would take eleven hours .. Do you really think an eleven hour drive would be sensible or safe ?

According to Google maps it’ll take 5h 30m to drive from Montreux to Füssen.

I stand corrected on the timing for driving . However , Google maps ( and Via Michelin ) can be overly optimistic about the timing . Taking into account road conditions ( time of travel - night or day ? , traffic , and lack of familiarity with the roads ) this doesn't seem practical to me .

Montreux to Füssen takes around 7 hours by train with a couple of changes along the way. And the driving times on Google maps are very optimistic. If Google says 5:30 it probably takes at least 6 hours in reality. The route also goes via a couple of major cities, and expecting the roads around Bern and Zürich to be free of traffic is probably optimistic.

I'm sorry, but Switzerland isn't the best starting point to see castles and WWII historical sites. Is there any chance you'll plan a Bavaria - Austria trip in the future? That'll be a much better match.
In the meantime there is plenty to keep you busy for ten days in Switzerland.

Maybe fly from Geneva to Munich, stay 1-2 nights, rent a car to see Neuschwanstein, Dachau and other attractions along the way?

I am considering traveling from Montreaux, Switzerland to Fussen, Germany in May ‘22 to see the Neuschwanstein Castle and Dachau...

I would encourage you to do some further research on places of interest in Germany. There are dozens and dozens of wonderful places that offer extremely easy access from your other destinations - and IMHO, the German destinations you are considering - if those are your only objectives - are not only too far away but also may only serve to reinforce the sort of cliched notions we Americans tend to have about Germany in the first place.

A few suggestions that are just across the border from Switzerland...

• https://en.wikipedia.org/wiki/Hohenzollern_Castle
• https://en.wikipedia.org/wiki/T%C3%BCbingen
• https://www.deutsche-fachwerkstrasse.de/en/Routes_Cities/From-the-River-Neckar-to-the-Black-Forest-and-Lake-Constance.html

Gengenbach + Black Forest:

• https://www.orte-bw.de/grafik/uploads/10557_2010_1040.JPG

• https://www.tripadvisor.com/Attraction_Review-g1825958-d190955-Reviews-Schwarzwalder_Freilichtmuseum_Vogtsbauernhof-Gutach_im_Schwarzwald_Baden_Wurttemb.html

• https://www.schwarzwald-tourismus.info/#googtrans(de|en)/

Balso, Russ and Nigel et all said it best. It will eat up so much of your time and be exhausting to do. Dachau is outside of Munich and Fussen is still a hike. The Neuschwanstein Castle was underwhelming to us. We got there, stood in a park, took some pictures from the outside, and left. A lot to go through just to say you've been there, done that. Visit that region of Germany when you have the time to do so. If Switzerland is your focus, there is SO much to do in Switzerland that will take your mind off anything else. Check out the castle in Thun. It's a lovely little town in Switzerland not all that far from Montreaux. The Black Forest area is another nice area if you want Germany. Again, posters here could fill your 10-days in Switzerland so quickly with suggestions of great things to do.
Save Bavaria for another trip.

All the connection I see using a night train take all night with only a few hours on the night train, the rest on regular trains - hardly worth it. Better to just do a "shorter" day route.

All trains to Füssen originate in Munich

Actually, there are very few direct (no change) train connections between Munich and Füssen. Almost all connections from Munich require a change of trains somewhere, usually Buchloe. If you are coming from Switzerland, a lot of connections come through Stuttgart, then thru Augsburg to Munich. If you get off in Augsburg, and go from there to Füssen, you'll save some backtracking.

Between 8 AM and 10 PM, 14 trains arrive in Füssen, 11 from Augsburg, 3 from Munich.

Almost all trains to Füssen originate in Augsburg.

I agree with the above advice - this is a really difficult addition to a Swiss trip that will eat a lot of time. And, if you are going to Montreux, you should visit the Chateau de Chillon which is a better castle than Neuschwanstein anyway - it’s actually a real one rather than a 19th century mansion.