> You just need rails to be able to quickly attach/detach a car
A single attaching maneuver takes 5-10 minutes (from personal experience). Add to that the detaching and driving the carriage off the rail to free it up, and you are quickly at 15-20 minutes, which is unfeasible for most long-distance train rides. Might work with short distances where you already have such a long pause planed in at the ends, but otherwise not so much. It also puts more strain/traffic on the track switch infrastructure at stations, which is already a bottleneck AFAIK.
Actually, on many RE routes the trains are nowadays getting fully automatically coupled/decoupled at certain places.
e.g. the RE7 Kiel-Hamburg and the RE7 Flensburg-Hamburg both arrive in Neumünster at the same time and are coupled together before continuing to Hamburg.
It takes roughly a minute to do the coupling due to modern automated coupling solutions.
On certain lines the ICE1 and ICE2 also did the same, and the IC2 is running the same rolling stock as nah.sh is using on the RE7 line, so that could also use the same technique for coupling.
But this all is very expensive and complicated, and in no way worth it just for coupling a wagon with batteries to the train
You just need rails to be able to quickly attach/detach a car, that's much less infrastructure than full electrification.