The German government has passed various measures attracting additional passengers through ultra low fare offers, largely against the advice of railroad professionals, and has failed to match these measures with additional capacity or investment.
This comes on top of an unfortunate history of problems with new trains and other equipment and lack of investment if not downsizing and minimalist thinking in other areas, meaning DB was anything but prepared for the additional riders.
I remember when they introduced those, especially when fuel prices were skyrocketing in Europe. I asked myself, so where is DB getting the additional trains from? Even if you want to argue new trains are on the way, and they have a lot of new equipment on the way, they're a few years away at best. So DB as some have stated basically shot themselves in the foot. As far as good rail service in Europe, the countries I hear about are Austria, Netherlands, Spain, and Switzerland.