>> 1. No seat reservation is required while travelling from Interlaken OST to Kleine Scheidegg via Wengen
Correct. Reservations are not even possible.
>> and there is no fee since it is covered with Swiss Travel Pass.
Travel with the Swiss Travel Pass is free as far as Wengen. From Wengen to Kleine Scheidegg, you get a 25% discount.
>> 2. No seat reservation is required while travelling from Kleine Scheidegg Interlaken OST via Grindelwald
Correct. Reservations are not even possible.
>> and there is no fee since it is covered with Swiss Travel Pass.
Travel with the Swiss Travel Pass is free as far as Grindelwald. From Grindelwald to Kleine Scheidegg, you get a 25% discount.
>> 3. Does Seat reservation is required for travelling between Kleine Scheidegg and Jungfraujoch?
No, seat reservations are not required. They are optional, and might be useful if travelling in July or August.
>> and I think there is extra charge for it in addition to the Swiss Travel Pass.
With a Swiss Travel Pass, you get 25% discount on the fare from Kleine Scheidegg to the Jungfraujoch. If you also get a reservation (most people don’t), that is extra again.
You can download a map from the link below, showing what is covered by the Swiss Travel Pass.
As explained in the legend on the map, red lines are trains, yellow lines are buses, black lines are cable cars, white lines are boats. Solid lines are free, dotted lines are discounted. Discounts are mostly 50%, but only 25% for transport where you see the number “1” inside circle near the dotted line. There are a few grey lines on the map as well, which indicate transport that is not covered by the Swiss Travel Pass. Dashed red lines on the map are railway tunnels, and all are on routes that are free with the Swiss Travel Pass.
http://www.myswissalps.com/sw isstravelpass/validity
Alpenrose