Which one? I use SBB and BLS regulary and i never noticed anything else than electric. (Really just wondering as i have no idea)
Don't get me wrong, it's not huge numbers, but I see 'em.
So, disregard my blathering, I stand corrected (hopefully!)
I traveled some times from Zürich to Munich and saw that in the German side, rails have no electricity and then the SBB are forced to use a diesel powered machine, but except for that, I still have to find a connection where the diesel machine is necessary every time, I am curious!