I saw the possible mock-ups of such a tube system (where everybody has to be seated) and had only one question: Where is the restroom and how do you stand up to walk there?
It's probably in the same place as the restroom in a car.
As the pods seem to be quite small, for longer journeys, it's probably possible/practical to request for your pod to be dropped at the next station on demand if you need to go. For shorter journeys, just hold it.
It goes down the tubes when it gets to the terminating station :) I'm sure they can design it like an airplane or bus cabin with regard to disposing of human waste, a small lavatory will probably be OK.
You have to think about children and older people who don't have the degree of self control we adults do.
Or the car itself will become the toilet.
P.S. Wondering who downvoted my above, and quite legitimate, comment. Somebody mustthink I don't like Elon Musk or something, when nothing could be further from the truth.
I have yet to see a restroom on a New York City subway train. Trips on those can take upwards of an hour depending on where you're going. Old folks and children seem fine. I don't think a lack of bathrooms is a very significant problem; I'd say it's not even a problem worth considering.
I hope Elon has considered this.