Hi Lorri -
Welcome to My Swiss Alps.
That will be a nice trip. I really like the slower upper route where the special train is named named "The Lötschberger." I ride it and get off at Hohtenn and hike to Ausserberg, where I catch another one of the hourly trains. to go on to Brig or return to Spiez/Interlaken./etc.
This link shows what the hike is like.. You'll go by most of it too fast to see all the fine the details if you are watching from the train windows.
I just checked the map for validity.
If you download it, you can zoom the *.PDF file to make it very clear.
Make sure to study the legend at the far right. It explains that the dotted lines near Wengen and the Jungfrau are routes where some discount is offered, but not a "free" ride. The numbers with circles around them refer to the details explained in thelegend.
The dashed lines on the map near Kandersteg signify tunnels. The heavier dashed line means that it is part of the main line routes which do not pass through Kandersteg. The lighter line is the old route, which does pass through Kandersteg.
The Swiss Travel Pass is valid on either route. You will not need reservations unless you are on the GEX from Brig or Visp to Zermatt..
The longer tunnel that does not actually go through Kandersteg is the new "Lötschberg Basis Tunnel" which goes through the base of the mountains and comes out near Visp. Because of the speed, it is the preferred route for travelers in a hurry to get to Italy via the Simplon. Most people find the scenery in the tunnel to be rather boring, which is why we recommend the old upper route, through (via) Kandersteg, which does also have a short tunnel. It also has quite nice scenery on each end of the route - that is, ascending to and descending from the tunnel entrance at Kandersteg and the exit at Goppenstein.
The upper route adds an hour +/-.
If you use the SBB timetable, you can compare by trying the route with and without "Kandersteg" in the "via" box.
The journey on the new fast out takes 2 hours and 42 minutes. There are some train changes.
If you put "Kandersteg" in the "via" box, you'll find that the trip takes 3 hours and 49 minutes.
If you wish to use the two routes on the two way trip, you should think about sunrise and sunset timing, for the best visibility.
the section from Spiez to Kandersteg has comparable scenery whether you sit on the right or the left. Te section from Goppenstein down to Brig is better if you sit on the valley side of the train. That is, on the right side ( facing direction of traval going to Brig, and th e left side coming up the "ramp" from Brig to Goppenstein.
This link is for Brig. Differences along the route will be insignificant.