A train system designed to reduce friction has better metrics in a vacuum environment?
Mars is NOT without atmosphere. It's about 1/100th as dens as that of Earth, but that's still not trivial.
The moon has an atmosphere, too. It's density is 13 orders of magnitude down from that of Earth, which makes it a pretty good vacuum. But that's because it loses air more quickly - on GEOLOGICAL time scales - compared with Earth. On HUMAN time scales, on the other hand, things like oxygen, nitrogen, water, and carbon dioxide hang around for quite a long time. (I THINK the half-life is longer than human written history.)
The moon's surface escape velocity is more than a fifth that of earth, and a non-trivial multiple of the speed of sound at ordinary (or even lunar) temperatures, so the molecules aren't just going to fly off any time soon. It doesn't have a magnetic field to protect it from the solar wind and its associated magnetic fluctuations. So over time scales compared to continental drift the Moon is leaky. But on human time scales it's not. Exhaust a bottle of air on the surface of the moon when your child is born and most of it will still be around when he dies of old age (assuming only current medical technology. B-) )
This has been a planning issue for those considering lunar industrialization. Much science fiction has industrial processes on the lunar surface, for the "cheap vacuum". But both the industry itself and human habitation in general will "Pollute the Moon with Air", quickly rendering the vacuum too "dirty" for such things (though still good enough to eliminate the need for a "roughing pump" for at least decades, if that's the ONLY source of new atmosphere). That would also be very good for evacuated-passageway transport systems like Hyperloop - PROVIDED they're only using motors, not compressed air, for propulsion. B-) But...
The length of time air would hang around has led to proposals to terraform the Moon, giving it a breathable atmosphere by crashing a LOT of icy orbital objects into it. (This, of course, would put it back to the Earth's situation. B-) )
While it would be back to a pretty good vacuum in time scales comparable to the evolution of life, it wouldn't decay substantially for time scales comparable to A life. Even if it started to become noticeably (without sensitive instruments) thinner in a few centuries, any space-going civilization capable of GIVING it the atmosphere would be capable of maintaining it.
I think the leakage is slow enough for civilization to fall - to stone-age levels - and rise again (maybe from descendants of apes) before it would be an issue, but I'm not sure: Looking up the half life of Lunar air loss on the net is a tad complicated: It's not talked about much. But the plotline involving an artificial Lunar atmosphere, in the game Half Life, is talked about a LOT. B-)