They are, however, laying power lines to have illumination of the entire tunnel. Seems more like they're doing batteries because that's something they have already, not because it's the optimal solution.
There's no doubt it will be less efficient, especially since the cars are carrying their batteries but the whole point of this is to cut CapEx by 90+% and eliminating miles of power lines is an important part of that effort.
It's a lower level of importance over this 3 mile trip, but it's not uncommon for power upgrades/maintenance to be a multi hundred million dollar Annual outlay for large transit systems.
A well-executed new metro line development, like the MetroSur line in Madrid, runs at around $60M per mile total cost including everything, all the construction and the trains and the stations and property rights.
If you want to improve on that by 90%+, you can't afford much more than just the batteries and the "skates" for this Dugout line.
- reuses technology Tesla have already developed and are familiar with
- increases sales of said tech
- a controlled setting where they can get more data about real-world performance of their tech
- introduces their tech to punters