Building Belts for one or two lanes is certainly more efficient than setting up 1-2 trains, but if you need to get lots of resources to lots of places, then your going to need dozens of lanes of belts that are likely going to need to cross over each other and are only good for one thing each, But 2 train rails with a few stops can do the exact same thing and more.Дon't forget each cargo wagon can hold 40 stacks of items, which is 2000-4000 items for most resources. This allows them to find a place with the raw resources they need, and then can bring over whatever they need to finish production. The biggest Let's players work around this by doing it modularly, so that all of the hard work is only done once and they have drones build all but the fine details. This of course is very difficult to accomplish, as it is very complex. On one single track, you can have any and all resources travel over it in your trains, but will never mix and always get to where you need them in a timely fashion. while you only had to build a few rails to do so since up to 90% of it will have already been done for hauling other resources. The copper will get from across the map from A to B, and it will only be copper. Set up one Big copper mine that makes copper plates, and then you can build a train stop anywhere connected to the network, and have a train haul only copper plates from that original stop to your new stop.

If you build one advanced network of train rails, you can then have dozens of trains use the same rails to get to different destinations. The major advantage trains have to belts is simply the fact that you can have multiple trains use one set of tracks to go to different destinations, Though you need to worry about collisions in that manner unless you build a more complicated system with 2 lanes.

Factorio train unloading