If one were to say... Want some discreet underground transportation. What options would be best? Mostly for people moving but some small freight. Also something quiet and power efficient.

What options are there?
What companies should I be contacting?

Of course I'm open to any ideas on discreet underground transportation, after all you are the experts.

Consider this a thought experiment for now. I'm sure we all have our ideas on supposed transportation scenarios we'd like to share. No?