Railroads are a special type of tile improvement in Civilization VI: Gathering Storm. They are the ultimate "road" in the game - iron tracks on which fast trains move with speed impossible for even the most modern road network. Railroads accelerate the Movement of units on land, allowing them to cross rough terrain and features such as Woods and Rivers easily. All units can move between tiles with railroads at the cost of only 0.25 Movement, and Trade Routes passing through railroads receive a bonus of 0.375 Gold per tile with a railroad.
Unlike roads, however, railroads are not laid automatically by Traders - rather, they must be laid manually by Military Engineers. Placing a railroad on a tile does not consume a build charge, but it does consume 1 Iron and 1 Coal.
Upgrading to railroads should be your ultimate infrastructural goal come the Modern Era. It takes just a single Military Engineer to accomplish this (although it will be a rather slow process with only one), and all you need is a steady supply of Iron and Coal. And then your land units will move through land with incredible speed, you will be able to react almost instantly to threats on opposite sides of your empire, and your trading will get a boost!
The only downside is that you'll have to use Coal, which will worsen your CO2 footprint. However, you do not actually need to upgrade every single tile of road to a railroad - just the strategic ways along which you cross your landmass. This way, you'll achieve the desired effect without spending an excessive amount of time and resources.