My professor of ancient history when I was in college had a project going with all his Graduate Students to compile all information on travel times in the ancient Mediterranean/Asia Minor (his specialty was Classical and Hellenistic, that is Post-Alexander Greece and Asia Minor). I don't know what the final outcome was, because they were still collecting data when I left campus, but I remember a couple of things they had already found out:
1. ALL the 'routes' of travel between cities were Trade Routes, in that they were used by caravans of pack animals or (more rarely) by wheeled carts. Local travel was strictly within 1 - 2 days' walk of a city, and simply wasn't important for travel between cities.
2. The exception to this general Rule 1, was the Persian Royal Road, which had been built strictly for express riders carrying 'mail' and government communications over 1000s of kilometers.
3. The other exception to Rule 1 was Greece, in which 1 - 2 days' walk would almost always bring you to another city, so there was little or no difference between 'local' and 'inter-city' traffic.
That means (as he finally winds up) that the pattern should be Trade Routes between cities for roads until relatively recently, but 'local' roads everywhere automatically - but probably, in game terms, only between the City Center and all its Districts. Traders and their Routes, though, were historically a lot more common than they are in the game: the relatively dense collection of cities/city states in Greece and even the larger area of Italy were covered by a network of roads, trails, tracks, and paths between every city, because 'long distance' Traders were Everywhere.
The problem, as has been noted already, is how to keep roads from being the default Landscape everywhere there is a Civilization.
Here's a suggestion that tries to 'thread the needle' between All or Nothing. Whenever a Settler founds a new city, it generates a road between the new city and the nearest city of the same Civ that is no further than X tiles away. X could vary by Era or type of Road, so it might be no more than 8 - 10 tiles for an Ancient Road (city founded in the Ancient Era), 2 tiles more for a Classical Road, and them jump to , say, 20 tiles for an Industrial Road and 25 tiles for a Modern Road you can go a long, long way on a multi-lane concrete highway compared to a pair of ruts across the turf!. Your specific Traders and Military Engineers, then, you could 'reserve' for filling in roads where you need them and connecting with foreign cities.
. . . you get an ME or maybe two and you just connect up your main cities with rails once you unlock. It's there but it's not that exciting to do. I wish these things could cost gold to place, like pay 100 gold to plop a rail tile. Then it would justify introducing some very fun effects of having cities connected. They don't even cost maintenance any more so only the difficulty of actively building roads is keeping us from paving every last tile.
Railroad building was so expensive that it required the development of stock companies and stock markets to raise the capital, and also gave rise to the massive chicanery and manipulation of the stock markets and railroad stocks that characterized the 19th century - see John Gordon's
Empire of Wealth for a summary of the process. That makes the building of railroads without Gold a complete distortion of the 'real costs' of the railroads.
This is another part of the Industrial Revolution problem in the game: neither the rising costs of the paraphenalia of the Industrial Revolution: factories and movement of population to work them, railroads, steam ships, massive enlargement of cities, etc. and the rising effects of Industrial Production: rising wealth of the middle class and upper class, rapid transport of Everything, mass production of goods relevant to the game like military, infrastructure, and Improvements, are not well modeled yet.
