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!