Amsterdam to Berlin by train

No it takes about 6 hours
https://www.nsinternational.nl/en

The direct train is perfectly adequate. The landscape is not really spectacular, so have a book with you.

There is only one train between Amsterdam and Berlin. This train is jointly operated by the Dutch Railways (NS) and the German Railways (DB, bahn.com).

there is a choice of trains, one being the German railway.

There are multiple trains, roughly one direct train every 2 hours. They are all operated as a joint service by Netherlands and German Railways (NS and DB). It takes 6 hours 22 minutes for the direct trains.
You can look up times and buy tickets (print-at-home) on either company's website. DB is here: https://www.bahn.com/en/view/index.shtml

For example, for a random day in April it is listing:

Amsterdam Centraal depart 05:02, arrive Berlin Hbf 11:22, 39,90 EUR
Amsterdam Centraal depart 07:00, arrive Berlin Hbf 13:22, 39,90 EUR
Amsterdam Centraal depart 09:00, arrive Berlin Hbf 15:22, 44,90 EUR
Amsterdam Centraal depart 11:00, arrive Berlin Hbf 17:22, 44,90 EUR

And so on through the day.