Trying to compensate for historic under investment now necessitates a lot of construction at once.

Nowadays Germany invests almost 200 euro per capita into rails (that's a lot by international standards and upper-middle-tier by European standards) and is a strong contender for having one of the densest rail networks in Europe. Investment would probably be even higher if they could figure out how to launch yet another construction project without causing the entire thing to grind to a full stop.

But investment and miles of track still relatively lag behind how heavily its rail network is utilized. Due to where it sits in the continent, ludicrous amounts of people and freight (especially freight) want to move through Germany's congested network everyday.

One one hand all the trains are late and very expensive, on the other hand certain operators would be happy if they got to move Germany's numbers even without getting to charge Germany's premium prices*. Depending on your point of view Germany's rail network is either suffering from success or being held back by terrible management.

* Ballpark freight numbers are that Germany moves roughly 4x France's volume of ton-kilometers while pulling in around 6-7x France's revenue.