I'll be traveling in Austria and Switzerland. I've read about the many advantages and perks or a Swiss Travel Pass. Does it work well on the Bernina Express routes? How wells does a 2-country Eurail Pass work instead of the Swiss Travel Pass? Does it cover basic travel the same but not cover small things like cable-cars and museums? Thanks!
As far as the Swiss pass is concerned, this map shows where it is valid: https://www.swisspasses.com/railpass/overviewmap_en.pdf
As you can see, it includes the whole of the Bernina Express route, trains, buses and urban transport in cities.
I am not familiar with the Eurail passes, but I understand it is trains only and not all of them, for example it is not valid past Interlaken on the route to Lauterbrunnen and Grindelwald.
Another option if you are doing a limited amount of rail travel is the half fare travel card. It gives ½ price travel throughout Switzerland for CHF 120 for one month.
Details here: http://www.sbb.ch/en/leisure-holidays/holidays--short-breaks-in-switzerland/swisstravelsystem/swiss-halffare-card.html
Both passes cover the full route of the Bernina Express. See a list of key differences under What do rail passes cover in Switzerland?