Eurail fully covers journeys until Grindelwald on one side and Wengen on the other. For the last bit you get a 25% discount (= around 150 CHF per person). It is the most expensive railway per km in the world...

Use jungfrau.ch or get the discounted tickets at counters. Prices are fixed and tickets cannot sell out.

I'd recommend getting them on the day when you know the weather is nice. Otherwise you're losing hundreds of CHF...

Thanks for the prompt response. 

QQ again: Do we get discount on seat reservation if we have Euro rail pass? 

Seat reservations usually cost the same. But in Switzerland, they're not needed and generally a waste of money.

No.

Which journey is a reservation required for ?

If you mean Eigergletscher - Jungfraujoch (10 CHF per person per way) it is too expensive for it to be worth it. You're already paying hundreds of CHF on tickets.

I have never paid such fees (and would never). Once I had to wait 30 min for the following train, the two other times I could board right away.

When you're up there, leave the main crowded building and walk a bit on the snow.