Simple...
It's fiction. You can have anything you want in fiction, including gasoline powered mat-trans units.
Clearly with the series there is infrastructure in many of the countries. After all, the Baronies got a lot of their goods from COG sites.
So, it would be difficult, but it could be done with the materials that were put into storage to repair, or even rebuild small scale refineries to process crude.
Not all tech was lost, nor the knowledge. It was only a matter of rediscovering it.
The simple fact is this...
You can argue the why's and hows of everything. I could, EASILY pick holes in the series if I wanted to.
Easily. But, because I'm friends with Mark, and because I enjoy the series, I can overlook things like that.
After all, how can things like, oh say, psionics, exist? It's been part of the James Axler Canon for years. Doomies, psi muties, you name it.
Can it exist in real life? I doubt it. I'm a huge skeptic. But look at Mask of the Sphinx, which I co-wrote with Mark... I took the city that he created for his DL novel and updated it to Outlanders, and made the entire noble class psionics.
Every one of them.
Now, because they don't exist in real life, hmm... I guess we'll just have to suspend our belief in what can, and can't happen.
People could EASILY say that it can't happen. And many people will. I've met people who won't read any type of fiction because they can't suspend their belief.
So, bottom line is this. You can think about things logically, and deduce that there is the infrastructure and the manufacturing capabilities in the future, thus suspending belief, or you can nitpick about it.
Choice is yours.
Chris