Rail Station in the Rhine Valley

I am assuming that you will be taking the train. I enjoyed the stretch from Mainz to Cologne. The pulse of the Rhine Valley is Rudesheim. Heading north, there are numerous castles worth exploring.
My personal favorite village is Assmannhausen, just north of Rudesheim, it is quiet, has a skylift that will take you to a hunter's lodge and from there hike to Germania a towering statue to view the whole valley.
Across the Hotel Krone, is Rheinstein, one of my favorite castles and the caretakers speak very good English.
Have a great trip. One of my fav. places that I go back time and time again.

Koblenz is the station you probably want. It is a larger station that serves as a hub for the area and lies just to the north of the most scenic and most visited part of the Rhine, where the Mosel River meets the Rhine. Smaller villages to the south include Boppard, St. Goar, and Bacharach; all have train stations too.

The Rhine valley in German is pretty long; it goes from Switzerland to Neatherlands. Are you talking about the middle Rhein gorge, where the river goes through a steep sided gorge with castles and wineyards? Koblenz is in the middle of that region, but the train lines from Berlin to not go to Koblenz (at least not directly or conveniently). You can go from Berlin either to the lower end of the Rhein at Köln or, more easily, via Hannover, Kasel to the upstream end near Frankfurt. From Frankfurt it is a short distance to Mainz, which is on the Rhein, and from there you can take boats (slooow) or trains to towns like Bacharach and St. Goar in the Rhein gorge.

Just realize that Germany is probably the most rail-acessable country on Earth. Without even consulting the Deutsche Bahn website, I can almost guarantee that you can take a train from Berlin to a major city in the west (Frankfurt, Mannheim, Stuttgart, Essen, Koln, etc). and catch a local train from there.