When you try to buy a ticket from Salzburg to Mürren on the SBB site you get this error message:

This error message is confusing. It is also wrong. There is no reason why buying a through ticket to Mürren should not be possible.
The tourist from Australia seeing that now panics. “How am I going to get to Mürren?” they think. Tourist starts looking at rental cars.
Earth to SBB: Get this fixed. This is probably the least helpful error message that I have encountered in years. And make sure that buying tickets to anywhere in Switzerland actually works.