I don't know why but your comment reminded me of something I hate about the current movement bonus that roads and railroads provide.
As it is, they reduce the cost of each move. But really, for infantry, there should be no difference between a classical road and a modern pavement. Walking is walking. Same idea for horses. Gallop is gallop, no matter the road (in fact, the horse would probably prefer the roadside grass/dirt). Thus, it follows logically that Civ roads make your infantry into motorized infantry. They're not walking faster, they're getting on trucks/chariots and driving.
Similarly, for railroad, it should make no difference whether you're a tank or infantry, because you're jumping on that train.
Therefore, whether it's roads or rail, all troops should get the same movement bonus. Armor with 4 points shouldn't get to move 16 tiles because cost of movement is reduced to .25 where infantry only gets to move 8.
Irl, it's actually harder to move armor on roads than it is motorized infantry (dramatically higher cost of fuel and greater wear & tear on the road, fewer manoeuvrable intersections, etc).
Consequently, roads and rail should provide flat bonuses to movement. Ex: +0.5 Ancient, +1 Classical, +1.5 Medieval, +2 Renaissance, +2.5 Industrial, +3 Modern, +6 railroad. Bonus is lost when you get off the road/rail tile.
These arguments would be true if we were talking about movements of individuals and individual vehicles, but in the game we are not: we are talking about Units of several hundred, thousand, or tens of thousands of men, animals, and vehicles.
Even Roman infantry had a donkey or mule for every 8-man squad to carry the 50-pound canvas tent, cooking pot, and food supplies.
Tang Chinese armies averaged 4 or more civilians hauling supplies for every 'warrior' in the army.
18th century European Armies had an average of one cart or wagon for every 100 infantrymen.
The 1942-45 US Army averaged 47,000 men in each Division Slice - and only 15,000 of them were in the division itself, and only about 7,000 of those were Infantrymen.
In every case, moving large groups of men means moving men, animals, and vehicles, and it is much, much easier on any kind of cleared space - paved or unpaved. Unpaved and rained on, however, has you marching, hauling, or riding through Mud which is Mother Nature's Giant Brake on all movement.
By the way, hard/soft surfaces were well understood by roadbuilders: the Roman Roads had a thin layer of dirt on top of the highest 'surface' layer of stones when they were built, to make the surface easier on pack or draft animals' (unshod) hooves and for the hob-nailed Roman sandals to avoid slipping on slippery stone surfaces and a rash of twisted or sprained Roman ankles.
Railroads are something else: the movement rate has noting at all to do with the rate of the unit, it is based on the capacity and efficiency of the Railroad and so should be a 'standard' distance regardless of the type of unit using it - but that distance might be changeable with later technology, since modern freight trains are both much heavier (10,000 tons is not uncommon) and move much faster than they did 100 or 150 years ago.
