There are use cases for battery trains. In remote, mountainous locations where the cost for electrifying a track is very high it is not uncommon to use electric trains with batteries. Here in Germany we have several regions where diesel trains have been replaced by them.
That is really a missing part of this whole thing. I get that I could build something myself, but I don’t want to have the hassle of doing it and keeping it working all the time (because I am able to build it, but not on a “works perfectly all the time” level). I really hope that sometime in the future there is a standard for smart speakers (and screens maybe) that allows me to add them to my cloud service of choice.