The Roman road system was very well built, and indeed wasn't superceded in quality until 1900. Almost an anomaly in ancient times. But, in game, the Aztecs' and Mongols' roads are just as good. There's no real differentiation in roads in the game, and I don't think it's really necessary. And I have contented myself with the assumption that "railroads" include modern highways. So if roads are going to be abstract, then leave railroads alone, IMO. But that's just my 2c.