Welcome to My Swiss Alps.
That can be a a very nice train ride.
You can study the route and timing options by using the timetable:
The timetable will be very helpful all the while you are in Switzerland, so study the instructions carefully.
There is a lot of information buried in it.
I have attached two screen grabs from the timetable for the journey from Interlaken Ost to Zermatt.
The fast route is what you get if you enter only the start and destination in the timetable. It takes between 2 and 2 1/2 hours. The timetable always picks the fastest route unless you tell it to go by a defined route, using the "via" box..
The second screen grab shows the route via Kandersteg. It uses the very scenic route over the older, higher route. It takes somewhat over an hour longer.
On each screen grab I have expanded one journey by clicking on the "+" to the left of the journey. That give details. In the foot notes you will see that you can get a map of the route, and other useful information, once you have opened up the details for a particular journey
You will see that on the scenic route, the special train is called "The Lötschberger."
I recommend that you take that route when you go to Zermatt, then possibly use the fast route on the return, but only if you are in a hurry then. The fast route enters a new, lower, tunnel under the mountains just after Spiez, and emerges at Visp.
The new tunnel is faster, but the scenery inside the tunnel is not very exciting, to say the least. Good place to take a quick nap. ;-)
Let us know if you have more questions.
By the way, when are you going? It matters for the weather in the high Alps.