The progression appears to go:
Ancient Roads
Medieval Roads (Military Engineering)
Industrial Roads (Industrialization)
Railroads (Steam Power)
Modern Roads (Combustion)
However Railroads come so close to Industrial Roads (it's the very next tech) -- and uses a different icon than all the other "route" upgrades -- that I have to wonder if there isn't something different going on there.
7
OMG the Civ Designers actually got a technological progression right for a change!!!
I am stunned. Stunned and Appalled. I expected the Spanish Inquisition, but not this...
Now, of course, it remains to be seen if they have examined all the ramifications of this...
First, if Industrial Roads and Railroads come close together, than they are probably equating the Industrial Roads with the grading and paving system developed by MacAdam at the beginning of the 19th century, only a couple of decades before the first railroads. But, since that was a rock/gravel paving instead of a 'tarmac' ("asphalt") surface, Roman Legions or Roman Engineers should be able to build them a couple thousand years earlier - the Roman rock and graded roads were very similar...
And, realistically, they would be the first roads that would give a real 'speed boost' because they are the first roads that eliminate the majority of delays due to weather, mud, etc. in travel.
They, like railroads and the 'modern' (concrete?) roads would require a maintenance cost, but their cost is low compared to the railroad and modern concrete interstate/autobahn-type roads' maintenance cost.
But, there is a real difference between Industrial Roads and Railroads - not that Civ games have ever gotten it right before, but the railroad is an Order of Magnitude more efficient at moving Industrial Quantities of goods - resources or trade - long distances. Just as an example, even on a well-made MacAdam road, your animal-drawn wagon can still only transport about 2 - 3 tons at 20 miles a day. You can put a lot of wagons on the road, but the goods are still moving at 20 miles a day. Even a primitive, early 19th century railroad can move 100 tons at 20 miles An Hour. The 'classic' railroad of the industrial late 19th century could move, per train, several thousand tons at 30 - 50 miles an hour, and move 10 - 30 trains a day over a given set of tracks. Only the modern high-speed multi-lane highway with tractor-trailers can come even close to matching the capacity and speed of the railroad.
So, hopefully, there will be some 'extra' effects from Railroads and late Modern Roads compared to Everything that comes before: more Gold from trade routes certainly, but also (ideally) a great increase to the speed with which city boundaries and borders grow - when you can commute to the city in minutes instead of days, the city tends to spread out, as a glance at the growth of land area covered by New York, London, Paris, and similar cities in the late 19th - early 20th centuries' shows.
At least they seem to have the basic depiction in the game for the possible realistic and gamingly interesting effects...