From to Interlaken to Lugano

The train back to Luzern is only slightly longer than driving, if at all. The Grimsel, Furka, and Gothard passes are an almost endless series of switch backs at 20 mph. The train is a relaxing trip where can actually look at the scenery rather than look for the next turn, which you better make or its a long way down. There are some really awesomely huge mountains in a direct line between Interlaken and Lugano. Going around them is about the only option.
There is another route, to Spiez and Brig to Domodossola Italy, then cut across on the Centovalli railway to Locarno, then a couple more short trains to Lugano. That can be done in 5 hours vs 4 3/4 hours through Luzern and Arth-Goldau.

Dan,

Especially with the Swiss rail network, the trip will be the most efficient and fastest route available even if there is a change in Lucerne. The fastest trip duration appears to be 4H:44M, which isn't bad at all. I doubt that driving will be any quicker, and likely won't be any cheaper.

I'll actually be staying in Murren, and I'm wondering if there is a way to connect from there to the Glacier Express train, and then connect from there to the William Tell Express to Lugano. Is that possible?

You can do it on trains that travel those tracks, but I would not worry about the Glacier Express or William Tell Express. The Glacier Express and William Tell have panorama cars, but also a mandatory reservation fees.
To get from Muerren to Brig to pick up the Glacier Express will require an early start (8 am). Then you will need to transfer at Andermatt to a local train to Goeschenen. There pick up one of the hourly trains to Lugano.
I think it would be not worth the effort. You would be on the Glacier Expresss for only about 1 1/2 hours. You can ride the same route on regular trains.
Don'r forget, the train from Interlaken to Luzern goes over the Brunig pass is ultra scenic. The train from Luzern to Lugano follows the same route as the William Tell Express, but skips the long boat transfer from Luzern to Fluelen. You could even add that in on your own if you wanted. The boats to Fluelen go at 10:12 and 13:12 from the Luzern train station dock and take 2 3/4 hours to Fluelen.