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You won’t find that information neither in the timetable (excepted some rare cases with mandatory reservations) nor on the ticket.
Every public transport route is assigned (mostly exclusivery) to a specific operator via a concession. Therefore there’s never any ‘competition’ between public transport operators on a specific railway or bus route. As opposed to e.g. civil aviation, where passengers often have some choice between ‘lesser’ (cheaper?) and ‘better’ airlines.
If you really want to find out for whatever reason, you could e.g. google a particular route/leg and hope that the responsible operator referenced that information on their website (e.g. Luzern - Interlaken => Zentralbahn). Without clear search results, the default would be usually SBB.
Or, as Krist already mentioned above, you could also ask here.
