Mar 20, 2018 - 3:35 PM
You're being very creative here with the options to save money :-) It's not totally clear to me where you want to go that day. Do you want to travel from Interlaken to Freiburg via Lucerne? Or is Freiburg just a virtual destination to get a cheap German ticket, and do you only want to travel from Interlaken to Lucerne?
As Rockoyster said, a regular ticket from Interlaken to Lucerne is only CHF 33 (€ 28). A German ticket plus a ticket from Olten to Lucerne is more expensive than that, plus you won't travel via the scenic direct route from Interlaken Ost to Lucerne: Golden Pass route. To save money, you can try Rockoyster's other suggestion to get a ticket from Interlaken to Freiburg via Lucerne from www.bahn.com/en. If you book well in advance they cost only € 19.90 too. But do check ticket conditions as Chantal said, and the route you're being offered (might be the Golden Pass route, might be via Bern or Olten).