I think there's a normal regio train that runs the exact same route.  You can go first class on this train and avoid most of the tourists...

Thanks Richard,  Is it better to stop over somwere and return the next day??

Now I'm studying the route and I realize that I only did part of it.  This post might be useful.  You can take a regio between Montreaux and Zweisimmen, then another to Spiez.  From there to Luzern via Interlaken I'm not sure about.

As for staying overnight, I guess it depends on whether or not you're OK with 10 hours on trains in one day.  =)

If you manage to do it all in one day with a day pass, you'll probably save quite bit of money.