In a medieval technology level Earth like world, a small widely dispersed group of individuals have access to technology at a level comparable to our near future (mid 21st century, hard science). For various reasons these people wish to live and remain hidden among their low technology neighbors and not draw attention to themselves or their advanced technology, at the same time they want to be able to periodically travel and transport small goods around the world to far off destinations in a reasonable amount of time.
What types of stealthy transportation systems should they be using to allow high speed transport for individuals or small groups, without alerting the locals that something strange is going on?
My current ideas:
Underground tunnels with high speed rail. This is obviously going to be incredibly labor intensive to build and maintain these types of tunnels even given advanced robotics, and there is going to be a long lead time for tunnel construction to any new destinations, making them more effective for shorter distances between commonly traveled locations.
Some form of high altitude or low orbit flying vehicles, possibly with a vertical takeoff capability, these are likely to be loud and highly visible during takeoff and landing (helicopter style or a rocket launch, neither is particularly stealthy) which could be mitigated by avoiding populated areas, but this seems like it would make them less useful if you have to travel by traditional means for days to/from isolated landing areas.
Some kind of submarines could be useful for travel in and around certain areas with lots of water, but the speeds seem pretty slow, in the 30-40 mph range.
Would these kind of transport types work? Or are there better alternatives I'm overlooking?
I am aware that a system of multiple types of transport working together might solve the individual problems and am open to solutions that use different modes of transport together in a functional system.
I'd like to keep the hand-waving to a minimum, so no teleportation, invisibility, or anti-gravity; just realistic technology available now or in the near future.