For Amsterdam to Berlin, forget the Interrail website: its reservations are too expensive (8€ per person and train). Use the DB website (4€ per person and journey) or ÖBB website (3€ per person and train) and choose "Seat Only”. Not sure if you can book this reservation via B-europe but it would also be too expensive at 6€ per person and train. If you still don't succeed via DB and ÖBB, please specify date and departure time.
For Prague to Salzburg, Interrail is also too expensive. What are the exact trains you want to take? (again, date and departure time)
I am planning to go from Ams to Berlin, either a direct overnight train on the 16/06 or earliest train in the morning 17/06. Prague to Salzburg 19/06, earliest train, Salzburg to Venice either overnight on 19/06 or earliest train 20/06, Venice to Rome 21/06. I cost me a fortune to book for 5 people for Lon to Paris first train on 13/06, Paris to Amsterdam on 16/06 and Berlin to Prague on 18/06, all first trains in the morning. Thanks for your help. I am looking at the above websites, and open to suggestions if there are cheaper ways of reserving, I can refund some of the reservations..
Eurostar is expensive at 30€ per seat; Thalys is comparably expensive.
Amsterdam to Berlin on 17/6 at 5 am is no problem on the DB website (4€, you can choose yours seats from a seat plan) or ÖBB website (3€, no seat plan). There's no direct night train.
Prague to Salzburg on 19/6 at 6:04 can be done via DB if you enter Freilassing as destination. It will then let you make reservations Prague - Linz and Linz - Salzburg for 4€ per person in total. (*)
Salzburg to Venice overnight on 19/6 can be done via ÖBB using these steps. The night train only leaves from Salzburg at 01:40, so you might consider to go to Attnang-Puchheim, Wels or Linz to enter it earlier. Or take the 8:12 train on 20/6, with a change in Villach. Again, you can book the trains on 20/6 for 4€ via DB if you enter Freilassing as your starting point (via ÖBB it's 2 x 3€ = 6€). (*)
Venice to Rome on 21/6 is best booked via ÖBB using these steps but the Italian summer timetable hasn't arrived there yet. Shouldn't take long anymore. You can already check the timetable on www.trenitalia.com (which is the first source for Italian timetables): the first direct train leaves at 5:26, the next one at 6:26. Reservation via ÖBB should be 10€ per seat (Interrail will charge 12€). There's no big rush though since these pass holder seats are not quota-controlled.
(*) DB will only let you book reservations if they can price the journey. And they can mostly only price the journey if it's from or to Germany.
For AMS->B and from Prahy (Where I was last week) via Linz to Sbg: its all OPtional, not mandatory, so doing it brings only a little peace of mind you will not have to stand. (which may sometimes happen if its really so crowded that all seats are taken-and you can always ask to remove all those luggages from seats. or if you see it-there may be a large youthgroup or school going out for excursions-take the next one.
For the direct trains Austria-ITaly there is also a 10€ SURcharge for passholders-if you pay that advance while in AT the REServation (which is also optional, except nightrains) is included.