Transportation and communication methods.
The Sime~Gen Chronology spans over 1500 years of Human Future History. The earliest written works start at the technological level of around 1750, with "incursions" of more advanced technology conspicuously taken for granted by the people of that era. So you have places where advanced transport takes on cargo brought in horse-drawn buggies.
These anachronisms are deliberate on the part of the authors, to indicate that though all knowledge of Ancient Civilization was obliterated, there is a current of connectivity that can't be permanently broken, and that when an advancement occurs, it will diffuse into the culture as irregularly as happened among Ancients, due to both economics and politics.
In the Sime~Gen Universe, the "astral plane" is "real" -- and so what has once been discovered leaves a permanent trace on the sub-strata of existence. Thus people in the Sime~Gen era can "retrieve" hints enough to fire the enthusiastic pursuit of recreating technology. Dreams, creative ideas, out of body experiences, inspiration, are all methods by which creative people retrieve ideas from the astral plane and add new ideas to the astral.
Simes and Gens do actively pursue archeology (in culturally different ways, toward different goals), digging up fragments and more hints of what Ancients knew and did, creating Householdings such as Frihil, museums and universities to organize what they find.
The sparse fragments don't so much give the Simes and Gens science and technology as they inspire them to re-think the entire conceptual structure behind science itself, generating an entirely new technology.
There are two determining parameters that direct this recreation of civilization. A) a sparsity of easily mined metal and B) the unavailability of easily accessed crude oil.
The 1800's of the Ancients saw the discovery of the usefulness of crude oil that was discovered in surface pools, or oozing from the ground. Pursuing ever richer deposits, drilling became the norm. By the end of the Ancient civilization, crude oil was retrieved from miles and miles underground.
By the time Simes and Gens consider the problem of retrieving any more of the crude oil that might be left, it would not be in any way economicly feasible to do so except possibly as a useful starting material for various medicines.
The Sime~Gen civilization develops energy use around the use of a) wood burning, b) organic oils (such as olive oil, corn oil, animal fat) and then c) selyn power.
Thus by the time a transportation industry is necessary, trains run on ceramic composit ribbons with cars suspended (without wheels) on an energy field created by the use of selyn.
Communications do not utilize copper (because it's just too expensive to retrieve from the oxidized remnants or to mine from so deep), or silver, for signal transmission. Instead, organic (gel like) substances are used to transmit signals using selyn power. This (ridiculously inefficient and dangerous) technology forms the basis of what becomes in the Space Age, known as Orgonics Technology.
By 132 Unity (Digen Farris's time) the slideroad trains cross the North American continent, cars run on selyn in-Territory and out-Territory, and during his lifetime, digital phone service spreads from in-Territory to out-Territory, with concommitant advances in computers (see Personal Recognizance ).
By 152 Unity, the real advent of the orgonics technological revolution begins in the Entertainment Industry, a legacy of Tonyo Logan at Unity, when ambient nageric fields are modulated and projected over large, in-person audiences. That discovery blends astral plane and science. (See The Nameless by Jean Lorrah.)
Simultaneously, as with most astral plane retrievals, A mathematical definition of the Levels of selyn storage (the equivalent of the invention of Quantum Mechanics) is devised: the initial publication contains errors. With revision, the math becomes useful beyond theoretical physics.
(Source: Index card file, House of Zeor) (-15 Unity) Horseback (with saddles), flatbed wagon, skis and snowshoes, sealed message tubes, "Tecton Weekly" newspaper, Horse-drawn buckboard --- a four-wheeled flatbed wagon, crude. Used for people rather than freight. Tourists to Hanrahan Pass are passengers in some kind of transportation vehicle. Grain wagons which are big and heavy. Carts are used in Iburan.