...also: your premise is slightly flawed.
The air on Mars is not "toxic", it is practically non-existent.
The air pressure on Mars is a mere 6 / 1000 what we have on Earth.
This works in your favour because with this rail becomes incredibly economic. The main advantage of rail on Earth is that you lower the rolling friction to near nothingness. The only thing holding a train back is wind resistance. So on Mars, with air pressure being so incredibly low, Elon Musk('s descendants) could build the Hyperloop without with the bother of building a vacuum tunnel around it.
Powering the rail is easy: you use electricity.
And why "no nukes"? You do realize that putting up a nuclear reactor is stupidly easy, right? If you are saying that the setting is "late 21st century", chances are great that fusion reactors have become commonplace. If Polywell turns out to work, then Mars will most definitely have fusion reactors. But even without that, having fission reactors will not be a big thing. If mankind have gotten to Mars and colonized it to the point you describe, then they will have solved the issue of energy.
Also: why would they be spread out so much, necessitating travelling back and forth all the time?! That is a completely illogical thing to do.
But no matter what justification you may or may not find for this premise... with too little air-pressure, that therefore cannot support aircraft of any sort; with no roads, no air, no fossil fuels and with scarce energy you cannot have personal transports (to any great degree); with no water you cannot have sea travel; and with the immense distances you need to travel, you need something that can attain great speed while using very little energy per passenger-kilometer.
Hence: rail is the way to go.