The Cattank appears to be electric, as is the Dalahästen train. The battery technology and electric motors in Adventure 1 are greatly advanced in comparison to ours, well within realm of science fiction.
If sufficient number of scientists and engineers have concentrated on the issue, 90 years (well, 80 or so, the Cattank is from the time of the Danish reclamation attempt) is a long while to develop something, although of course even at best the resources they can spend on it are quite meager and a lot ot the existing knowledge should have been lost. Still, Iceland has near unlimited energy from geothermal, so learning to store and use electricity in many ways would make sense.
All and all the technology level doesn’t seem to be 100% coherent. Of course with the enormity of the destruction it was also pure chance what fields endured better than others, depending on the survival of just a few or even single individuals.
I also have some ideas growing from the petroleum discussion: petroleum mines. Say a big harbor where the storage tanks were left, some have exploded but some remain. Work there is dangerous and dirty, but either a) pays very well or b) is done by a slave workforce. I have to say my train of thought is on track b and it’s a very dystopian idea. Maybe it’s somewhere else than in the Known World? Or maybe it takes place between Y0 and the current apparent stability of Y90?
I don’t think I will ever write this, I like SSSS partially because it’s not dystopian despite everything. And if I want to go nasty, I have another idea that’s been in the back burner for years. But maybe someone else is intrigued by the idea? All in all there entire “post-apocalypse junk rat” genre is waiting for the writers, largely untouched.