Can I Show Voucher for Glacier Express from my Phone?

You are literally talking about printing one sheet of paper.

I'm sure that is true. I was just hoping that like boarding passes on the airlines I could do it electronically so I don't have to keep up with it.

I'm 99% sure that you can. Last year I bought a ticket on my SBB app for a local train, and the conductor scanned my phone display just like a regular ticket.
HOWEVER, I agree with the other comment about printing it out, because:
What if you phone battery runs out that day?
Your phone is stolen?
Your phone gets wet in a rainstorm and won't work?
Your phone gets stuck on some app and won't show what you want when the conductor comes around?

The paper ticket (usually a half page) can be folded inside your passport, and that should be next to your body. If it isn't, you haven't been reading Rick Steves carefully enough.
If it was me, I'd print it.