Train reservation Basel to Murren?

Other than some of the special scenic trains with the panoramic windows which can get crowded in the summer, you really don’t need reservations on regular Swiss trains. For the route from Basel to Murren you can just buy your tickets at the train and gondola stations.

No need for reservations.

You buy the whole trip together and use whatever trains suit you through the day.

Even the French TGV all reservation trains do not require reservations in Switzerland.

If you are concerned about not fitting (I wouldn't be) check the app which will tell you (from 1 to 3 stick figures, least crowded to most) what the projected loadings are on each leg of the journey.

In Switzerland a crowded train means you might be sitting next to a stranger. It is rare, especially on long distance trains, for all seats being occupied, and some people needing to stand as a result.

SBB is an excellent railway. They run frequent trains, and will run very long ones if needed. It is not uncommon to see 16 car double deck trains on some services. Swiss people never reserve seats.

BTW: Even though this trip will involve trains operated by three companies, and a cable car you can do this on one ticket. Just buy a ticket Basel - Murren (or if you are starting somewhere else, from there).

3 of the connections from Basel SBB are actually long distance trains originating in Germany. They are marked as ICE or EC trains, and they are the ones at 11:55, 12:25, and 13:55. Perhaps on a Friday or Saturday in the summer they could be crowded. As Nigel suggested, check on line or with the app on the expected loading on these trains. You can purchase seat reservations for the Basel-Interlaken leg from SBB for 5 ch per person.
https://www.sbb.ch/en/travelcards-and-tickets/tickets-for-switzerland/seat-reservations.html

The ICE services are actually likely going to be less crowded than regular IC trains, because these trains are rather long. In my experience not even during rush hour do those trains leave Basel completely full (I commuted to Basel daily for three years...)