So I just came back from Jungfraujoch...
I have B-O pass and 1/2 fare card - so the fare is $75 with both..
I tried the SBB app - and from Eigergletscher to Jungfraujoch - it is $75 RT w/ 1/2 fare card (same price)... and the train originates from Kleine Scheidegg..
So I went to Kleine Scheidegg before purchasing the the train tickets..
Around 10AM - there wasn't much crowd - and I decided there is no problem getting on the train...
I immediately opened the SBB app and purchased the Eigerglestcher to Jungfraujoch RT right there waiting on line..
Since B-O covered the train to Eigerglestcher - the SBB app worked perfectly.. Early morning there were lot of empty seats on train..
Returning around 445PM - the crowd got noticeably larger with large Tour group getting the priority boarding.. We finally board after hoard to tour groups and reserve ticket holders got on first... and people were converging onto the train pushing and being aggressive... we finally did get in and got seats.. but train was full returning back to Kleine Scheidegg..