Actually, having bothered to pay more attention this morning on the way in, I noticed that the engines that I assumed were diesel were in actual fact electric; I had just assumed diesel from the shape and behaviour.
So, disregard my blathering, I stand corrected (hopefully!)
me too, I travel regularly from Basel to Lugano, daily in the Bern - Zürich, also using regional trains and never saw a diesel machine for passengers transportation, except for some rare unexpected cases.
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!
Actually, due to increasing traffing from Switzerland to Munich, Deutsch Bahn decided last year to upgrade and electrify the connection from Lindau up to ( I think ) Geltendorf.
Don't get me wrong, it's not huge numbers, but I see 'em.