Trains are pretty cool, and have some upsides and some downsides vs belts
Belts are simple, can be extended, and work pretty much anywhere. I think the price adds up over long distances however, especially with high level belts, and if you want large volume.
Train tracks are far cheaper than belts, can handle many material types each, since you can run several trains over the same section.
Each train car can, at the station, load/unload, I think, 288 items per second into 12 chests with max inserter upgrades and stack inserters. Before, it was 48. In comparison, a single blue belt has a rate of around 40 items per second throughput using both sides.
This is per train car however, and you probably want to have at least 4.
Price wise. you get 2 rails of train track for 6 iron + 1 stone, covering a distance of 4. For the same distance, you need 4 express belts, per row you want to transfer. Each one costs you 21 iron and 2 lubricant, meaning 84 iron and 8 lubricant. That is WAY more expensive.
If you go with 2 way rails, or even a 2 and 2 rails, you still only go up to 12 and 24 iron per distance of 4, so it is still cheaper.
The upkeep. Trains use up burnable fuel, whether that be solid fuel, wood, or coal, or anything else. I don't really have the math for it, but I doubt it uses more than 50 coal per trip at most, which should contain 8k ore. And you should be sending several trains on the same stretch of track too.
Last thing is, you can ride the train, meaning you don't have to walk the distance, and it is faster than something like 6 exoskeleton equipment.
Belts cost more, but no upkeep, and simple. However, setting it up is tedious, and having to have 4 or 8 lanes of belts is honestly ugly.
Trains cost less, but need fuel, and have massive potential throughput.
my math is probably not entirely right thoughput wise. I only guessed as exactly how fast the train loads and unloads, assuming a few numbers. Pretty sure stack inserters are 12 per swing, and 2 swings per second, which should add up. Also that assumes you are always loading and unloading, which is inaccurate. And you can have more or less train cars.
That said, trains are far easier to build and use now, so experiment.
viewtopic.php?f=5&t=18903
Apprently there are other discussions on this subject. Not surprising. Probably should have searched and pointed instead of mathing.