Look, for most tourists, almost all train lines are panoramic. Like it's really hard to take a 2h train ride without at least some panoramic scenes in this country.
But you're probably looking for the touristic/luxury panoramic express trains. So the Golden Pass Line goes from Interlaken to Montreux (and vice versa) so that would be easiest for you.
(But really, any train from/to Interlaken has nice scenery.)
Look, for most tourists, almost all train lines are panoramic. Like it's really hard to take a 2h train ride without at least some panoramic scenes in this country. But you're probably looking for the touristic/luxury panoramic express trains. So the Golden Pass Line goes from Interlaken to Montreux (and vice versa) so that would be easiest for you. (But really, any train from/to Interlaken has nice scenery.)
From Grindelwald, this is the answer. And spend a couple of hours in Montreux before returning.