Welcome to my Swiss Alps.
You can find a coverage map for the Swiss travel Pass in the links under:
Basically, you understand the coverage correctly. A rule of thumb is that if you are traveling between population centers, the travel is considered to be public transport, and is covered. If you are on special or privately run trains, such as the cograil from Wengen onward to Kleine Scheidegg and Jungfraujoch, and many "ski" lifts ( cableways) you may get a discount from the reguilar price. In the case of thethe Jungfrau, I believe it is 50%
There are surcharges for some special trains, such as the Glacier Express. However, "regular' trains also run on that same route. There are small charges for seat reservations on regular trains, but they are not needed. When you buy a ticket (or use a Swiss Travel Pass) from Zürich to Zermatt, it is good for the day that you buy it on anmy train on the route. if you happen to miss a connection, such as at Bern, or want to break the trip, no problem. Just take the next train.
If you use the timetable, you can learn a lot more about rail travel:
Read the instructions page at that link before you try to figure out costs for tickets if you choose to buy point-to-point tickets instead of a rail pass.
The beginning of April will be late winter or very early Spring at low altitudes such as Interlaken. At high elevations ( Zermatt, Wengen, Kleine Scheidegg) you'll have Winter weather and snow blocking high trails. It can be cold at Interlaken . By late April, it will be better, but you'll be lucky if you have warm weather.
Check the links under:
Did you check this link?