It's not simple with the "clock-face scheduling" system which is used which times the trains to all meet at the big nodes (Zürich, Bern, Basel) so connections work. To achieve this trains are supposed to fit into 30/60/120 minute beats which synchronise the entire system. See [1,2] for how this works.

Also many of the most important parts of the system are at capacity. Bigger trains can help but a lot of these gains have already been realised in the crowded areas. The current hope is digitalising signaling to allow density to be increased but that's not simple/cheap even if it's cheaper than working on the lines themselves.

[1] https://en.wikipedia.org/wiki/Clock-face_scheduling

[2] https://www.youtube.com/watch?v=OMbV1rIPhCg

You can't add more trains if the schedule is full to the brink. You would need to add train tracks, and that requires big projects

Frequency is basically 15 minutes almost all over the country already