Yes, it is possible you would not have a seat for some of the journey but not that likely midweek in April.

www.bahn.com will give an indication of how full those trains are likely to be and you can book seats there if you wish, use seat only option.

Alternatively www.oebb.at ,if you have 1st class passes reservations are free by adding your pass as discount and searching for tickets not seats

Thank you for your reply.

I checked bahn.com and the train I want its written “reservation impossible” for the seat but its also medium demand expected so that means all seats are taken? Or how can I book a seat at this trip? From station or just risk it? 

What is your travel date and departure time so we can have a look ?

Regional trains usually can’t be reserved

Tomorrow 25 April from Amsterdam Centraal at 20:28 - frankfurt (main) hbf at 02:11 am

then from Frankfurt (main) hbf at 2:25 am - salzburg hbf at 9:21 am on 26 April 

There are 3 trains on your journey :

• Amsterdam - Frankfurt 20:28 - 02:11 reservation not possible but as it has 7 carriages, I don’t think it’s necessary
• Frankfurt - Munich 02:25 - 07:28 hundreds of seats
• Munich - Salzburg 07:49 - 09:21 no reservation possible or necessary

You’ll need to use 2 pass days for this journey as you board a train after midnight.