Seat reservations are optional/not possible on Swiss trains and nobody bothers. Hop on and sit wherever you want. As a tip, the front end is generally less busy. But anyway the train will be empty at 6am on a Sunday. :)
(Side note: PE2937 would be the 18:04 departure from Interlaken Ost.)
Seat reservations are only required on the Glacier Express, Gotthard Panorama Express and panorama carriages of the Bernina Express. There are always hourly reservation-free alternatives.
@thibcabe Thank you for the quick response. You are correct. The PE 2912 is the correct train from Luzern at 06:06 on December 22.
Last question: I reserved seats (Prestige) on the Goldenpass Express in Interlaken OST at 9:07 am. Should I take the PE2912 from Lucern (06:06 - 07:54) or do I have enough time for PE2914 (07:06 - 08:54)
The later train will be fine. It’s a legal connection, and quite comfortable.
Take the extra hour in bed!
You will also get some daylight :)