Fundamentally, the Germans tried to cheap out on high speed trains by trying to engineer them to run on normal tracks so they didn't have to build new, expensive high-speed tracks like France and Japan did.

This is the fundamental mistake underpinning their train service since the long distance trains frequently have to wait for other trains to pass, cascading delays through the system.

That and an almost criminal level of underinvestment in the past 20 years or so.