<<"does this mean it doesn't require advanced booking, for let's say going
to Jungfrau or Lauterbrunnen? Even when I plan to go to the alps
(Jungraujoch / Schlithorn)? (I notice how I need to change between
trains and etc).">>
Reservations are not required on any regular or routine public transport Swiss train inside Switzerland. That pretty much means trains between population centers. International trains require them outside the Swiss borders. An example - travel to Italy from Zürich only requires them after you cross the border..
When you buy a ticket fromZürich to Interlaken, as an example, the ticket is good for any train on that route for the day of ticket validity. If you buy from a machine, you are given the choice of "today" or "future date."
The "future date" used to be good for a month...useful for round trips. Have not gotten one recently to check. I travel with a half-fare card and buy my tickets just before I get on the train. If I am catching a very early train in the morning, I buy my ticket for the correct day on the night before.
Certain special panoramic trains require reservations, even though they go from city to city. . Glacier Express comes to mind. Also, the special seating panoramic cars on the MOB portion of the Golden pass Route that runs down to Montreux require reservations.
www.myswissalps.com/ac tivities/scenictrips/t rain
Some will not run until late May.
However, even when there are special or named trains on routes that run on regular tracks between population centers, regular trains also run frequently and do not need reservations. You can tell from the timetable. Even the back cars of the Golden Pass Panoramic don't usually require reservations, although they do no harm. A ticket for a journey from....for example, Zürich to Lauterbrunnen or Wengen does not require reservations. Even the excursion train from Wengen up to Kleine Scheidegg (continuing from Wengen) which is a privately run excursion railroad, does not. To go from Kleine Scheidegg to Jungfraujoch does require that you book seats. Unless the crowds are very heavy ( not likely in March) you can buy the tickets right at Kleine Scheidegg.
It is best not to book to Jungfraujoch too far in advance. The weather in the mountains changes rapidly, and forecasts are not good for more than 24 hours in advance. The Alps make their own weather. You do not want to pay the expensive price for the Jungfraujoch train, only to find that the only sights are the inside of a cloud. Boring and expensive. However, the weather come sin layers. it may be clear up top and foggy down below. Ask at the manned stations nearby ( they may have a webcam) or your hotel, or the tourist office.
No reservations needed for Schilthorn, no matter which route you take. Most of that route is by cableways,