In civ 5, units consumed resources while they existed. In Civ 6, you merely have to have the resources while the unit is being built or upgraded. I've always thought a nice middle ground would be that the resources are consumed while the unit is being built. In this context, maybe you only need 1 iron to build a railroad, but that iron resource is consumed for a single turn when you select the option to build it. If you wanted multiple engineers building railroads across different parts of your empire simultaneously, you'd need multiple copies of iron. I always thought it would be nice to bring back an aspect of having a reason to have multiple strategic resources.
It's a little off from the Thread OP, but my Ideal Resource System would divide all 'Resources' into Ordinary and Industrial Quantities. Almost all early 'Luxury' or 'Strategic' Resources would be Ordinary: the quantities required to make a difference are relatively small. 100 pounds of Spices would make you rich, and therefore be worth trading, less than 200 tons of Iron would equip an entire Roman Imperial Legion with weapons and armor, and it's only needed in 50 pound 'blocks' (amount required for one Heavy Infantryman).
That means, all the Ordinary Resources can be traded by virtually any route: back pack, Llama-back, cart or dug-out canoe, and so would be in the Trade System from the start.
Industrial Quantity Resources are those required in quantities too great for 'ordinary' land transport: Food, from the start, because it takes 1000s of tons of the stuff to feed the 1000s of people in a city for more than a day at a time. Most of these quantities become important - you guessed it - in the Industrial Era, when instead of 200 tons of Iron for one Legion, it takes that amount for 3 kilometers of single-track, low-capacity railroad (rails, spikes, the amount does not include what is additionally required for the locomotives and cars to use the railroad).
A single Ironclad would require 1000 - 3000 tons of Iron, and now we're talking about 100+ ton sheets and structural members, which will NOT fit on a Llama or any other beast of burden or wheeled cross country vehicle of the time.
So, the concept is, early Resources appear on the map, and you can trade Ivory, Spices, Dyes, Copper, Gold, Iron in the quantities needed for swordsmen or happy civilians. To trade Food, you need to be able to trace the Trade Route by water only (rivers, coast) because nothing else available will handle the quantities required to make a difference at the game's scale and timescale. (This, coincidentally, would automatically increase the usefulness of Coastal Cities, which become much easier to 'feed')
About the time you reach the late Renaissance - early Industrial Era, 'tags' start to appear (based on separate Technologies like Deep Mining) indicating that some of your Natural Resource deposits are Huge - 1000s of tons instead of 100s, and accessible in huge quantities (close enough to the surface).
So, basically, you might have enough iron to equip all the Swordsmen or Knights you want, but not enough for your first railroad or Ironclad, and definitely not enough for Battleships, Tanks, or Modern Era skyscrapers, bridges, and other Infrastructure.
The Good News is, with advancing Technology, resources previously undiscovered or not shown because there was no feasible way to access them would become visible on the map for exploit.
Or you could end up like Rome/Italy, with plenty of iron for Legions in the Classical Era, but not enough Iron or Coal in Industrial Quantities to support the needs of the Modern Era (see: Italy in World War Two, a depressing field of study!).
And, to return to the Point of the Thread, Railroads would require both Gold and Iron in Industrial Quantities, but the Gold is abstract, and can be provided by new Economic Measures: the Corporation, the International Stock and Bond Exchange, etc.