Why should this setup be locked in to any particular third party company? How different is it to the train cars themselves being built by some company and then

Why should this setup be locked in to any particular third party company? How different is it to the train cars themselves being built by some company and then maintained by train company mechanics? And battery + electric motor setups are famously pretty straightforward, so this seems even more true. I don't disagree on the regulatory stuff being an issue, that's a very good point. The solar mesh feels like an orthogonal problem, also feels very solvable with just standard interfaces which would surely be mandated. I mean the solar bit is an add on too, you can always fall back to other means for battery charging, but seems like solar is just an obvious economically optimal solution in the circumstances.