Hey, for Switzerland you don’t Need any reservations (except for some touristic panorama trains, but there are regional trains on those routes without supplement/reservations).

Also for the trains in Lauterbrunnen area no reservation needed. Just hop on the train.

Reservations in Switzerland are a waste of money

@MartinM The trains in the Jungfrau-region are not included with 100% discount but you get 25% discount with Interrail/Eurail. 

@kristinerbas Yoy need to get a ticket with the Interrail/Eurail discount to travel in the Jungfrau-region. Just go to an SBB ticket office, show them your pass and say when and where you want to travel. Be aware that the tickets are very expensive. You don't need a seat reservation but ask at the ticket office if they recommend you to make one anyway if that is possible. 

OR -if you plan to travel there more extensively-there are also various local passes-which may save a little-nonetheless prices will remain sky-high, as befits this mountainous area.

Swiss has an awful long list of for outsiders confusing systems and companies-there is of course an efficient list of who takes what in the general info- but it still needs often guidance. As a very rough rule ecpect that ALL those specific mountain lines are OUT of the pass.

Jungfrau railways have an excellent website and some very good value holiday passes (no discount for interrail etc) You should reserve to go up the Jungfrau though because on clear days you will need to Queue at Kleine Scheidegg possibly for a long time. You can do this at Jungfrau railway offices. The trip up the Jungfrau is the most expensive train ride in Europe so choose a clear day and check the forecast!!

Sorry - the queues going up the Jungfrau are at Eigergletscher. I was there a few weeks ago.

Thank you so much!