May 16, 2013 - 10:46 AM
Hi gkumar, and welcome to MySwissAlps,
Don't worry, all posts will be answered sooner or later. Your Eurail Pass
will indeed get you a 25% discount on the Jungfrau railways. Tickets can be purchased at any manned station in that region. Just go to the ticket counter, explain your plans and they'll provide you with the most convenient ticket and include your discount.
I don't know where Kleindsteg is. If you mean Kleine Scheidegg, then yes, you can purchase tickets there too.
Enjoy your stay!