You might need an onward proof of travel of leaving the Schengen Area. Germany and Poland are in Schengen so basically the seat reservation is a useless proof. There are no border checks whatsoever between countries in Schengen. You'd need a proof of travel to the UK, Ireland or any other country outside Schengen.

For this train it's best to book when you arrive in Berlin.

Btw Eurail isn't the best place to book seat reservations as they add a 2€ booking fee per person per train (+10€ fee for paper reservations while they sometimes can be obtained online free of charge). See general advice :

https://www.seat61.com/interrail-and-eurail-reservations.htm

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Thanks for the quick response.
I have a flight that is from Dubrovnik to Serbia but that is in July, would this be enough even though I am still yet to book some train tickets through the Schengen Zone?

I am more then happy to purchase the ticket in Germany I am just a little worried about the proof of onward travel aspect. 

I don't see how a train reservation can be a proof of you leaving a country. Anyone can get a train reservation and it might not even have your name on it. And there will be no record of you actually using the reservation. 

Yes obviously the Dubrovnik - Serbia flight is the proof you need. Until then you have 90 days to explore the Schengen Area.

And as Anna said a train reservation does not really look like a proof but then even a flexible plane ticket could be refunded so I guess it's more of a trust system

Don't worry and make your seat reservations along the way, esp. in Eastern Europe where they are rarely required. Night trains on the other hand should be booked in advance