I've used train networks quite a bit since 4.1 and they are awesome. Some problems I've encountered:

1. It is difficult to get trains to refuel when they should, because they only "check" for fuel after a "job" ends. If, for example, you want your trains to refuel when below 30%, they seem to only check this after a complete load-unload operation has ended, never after a load, for example. Two possible solutions I can see:

• Allow us to check a box if we want trains to check for fuel after visiting any station, load or unload, or just at the end (how it is now)
• Even better, a smarter solution would be to estimate how much fuel an operation would require and decide if a refuel is necessary before the operation begins. It doesn't have to be super precise, you can consider a margin of error. For example, if a load-unload operation should take 25% of the tank, but the train has 27%, it can still go refuel (a margin of error of 5-10% can be considered). The problem here would be that you have to consider the distance to the fuel stations as well, which maybe adds complexity to the calculation.

2. You can currently select which train locomotives are allowed at certain train stations in a train line, but the same doesn't seem to be possible for a station in a train network. I had a diesel + hydrogen network for spreading fuel across the map, and I had diesel and hydrogen trains. It would be optimal to have the hydrogen train transporting hydrogen, and the diesel one diesel (since they can refuel while loading the cargo), but this doesn't seem to be possible, unless you make two separate networks, of course.
3. Selecting how much cargo loads and unloads. My understanding of the base premise of networks is that they allow trains to be used for different cargo loads, so you don't need, for example, an iron train, but rather a loose train that has a job for one product, then the next etc. Of course, for this to work, the premise must be that whatever is loaded gets unloaded, because if a train has some cargo remaining, it cannot go pick something else. But it doesn't mean that a train must fully load, just unload. Where this was a problem for me is for unload stations that are smaller than load stations. For example, I had a "big" limestone load (12 modules) right near the mine, and big unload (also 12 modules) for my main factory. This was serviced by 6 T2 wagons, as would be optimal. Well, the gripe is I also had a smaller limestone unload (4 modules) for another part of the factory. If a train with 6 T2 wagons takes the job of unloading there, it will load all 6 wagons and take forever to try to unload all the limestone at the small, 4-module station. My solution was to just build a smaller train, with 2 T2 wagons, for that job. But it seems obvious to me that the large train should have understood that it would be optimal to load just 2 wagons and unload them fully at the 4-module station. Curious to see what people think, there may be use cases where this is the opposite.

Kaiser wrote:

Well, the gripe is I also had a smaller limestone unload (4 modules) for another part of the factory. If a train with 6 T2 wagons takes the job of unloading there, it will load all 6 wagons and take forever to try to unload all the limestone at the small, 4-module station. My solution was to just build a smaller train, with 2 T2 wagons, for that job. But it seems obvious to me that the large train should have understood that it would be optimal to load just 2 wagons and unload them fully at the 4-module station. Curious to see what people think, there may be use cases where this is the opposite.

One thing I've noticed about these kinds of situations is that network trains will never load more cargo than can be received- much like trucks. My gripe with these less-than-full-train stations is that the train won't dispatch until the station and connected storages are completely dry, meaning that whatever it's connected to is being starved for the entire duration of that train's job time. Or for pickup stations, where the train won't dispatch until it's completely full... which means that for low-volume critical products, I basically have to build enough storages on the station that it can fully load/unload an entire train, plus a little bit of extra "buffer" room for the travel times. Especially when I'm using a temporary, shorter-than-the-train station.