Sometimes youll need stuff like this. Rail maintenance cant always be done using overhead lines, since the machines will get destroyed by electric breakdown(?).
Some parts are not electrified yet, some cant be without major work being done to the track.
It is not ideal, but sometimes you cant do it otherwise, or you’d have to cut of some parts, imo its a useful way to bridge gaps.
You can totally go 250km/h on the A5 from Frankfurt to Darmstadt, if traffic allows it. That wont always be possible, but typically if it isnt during work traffic, 160-200km/h is doable comfortably, even on other parts of the Autobahn
The longest unrestricted part that ive driven should be the A71, where you can go longer passages without restrictions, save for the tunnels.