Background: If we look at the past (especially in countries from the early C.E.), a lot of empires collapsed for many different reasons. These reasons are mainly (not exclusively!) going to be an economic collapse, lack of fast communication, corrupt leaders, and/or rebellions among the common folk. However, in this world, only one of these problems poses an actual threat, that being the lack of fast communication.
Context: As most empires grow larger, their lack of fast communication is one of the main few reasons they will fall. In an empire I'm creating, they do not have access to electrical communication, limiting them to slower forms of communication from one end of the country to the other. Let's say that this empire is going to be about half the size of the modern United States. This empire would need fast communication in order to discuss important matters between two or more leaders, along with spreading the word and enforcing laws/rules that have been put into effect. The technology they would have available that may affect the answers is as follows:
- Combustion engines as used in the mid-late 1800s America (meaning trains and very early versions of cars)
- Well-paved roads (flat, smooth, good for travel)
- A printing press (similar to Gutenberg's press)
- Train tracks consisting of about 40-50% of the distance needed to cover
Question: What does an empire with this kind of tech (or any others that you may come up with) use to the best of its ability, to keep up fast communications across the country?