Stations can only unload so fast, no amount of trains will help you if they all queued behind a single station, everything is pretty much question of scale.
For example, making regular ammo, it takes 4 iron plate per banana, so in order to get 320 bananas per second (8 blue belts worth), you will need 1280 iron plate per second. Single station can't handle it, you need 32 blue belts worth of iron plate, so you need four (1-4) train stations to get 32 compressed belts.
Something like this:
- factorio_ammo.jpg (582.25 KiB) Viewed 4220 times
This setup produces 320 bananas per second, and eats 32 belts worth of iron plate when running at full capacity. If all the stations were completely full (2.6 million iron plate), it would take 33 minutes and 20 seconds for them to go completely empty, to keep up with the demand, you need a train delivering plate (32k per train - 2x normal train cargo size) every 25 seconds, or every 100 seconds for each station if you consider only these 4.
With turnaround time between iron forges and the station about 50-60 seconds, that same station will send another request by the time the first train has a chance to come back. And so will the 3 other stations.
That's pretty much it for the scale.
Handling request queue of 7-8 trainloads is no problem, there are 16 trains delivering iron plate at the moment, since the factory is not even third finished, most of production shuts down after the initial rush to fill the outbound stations when i add new segment to the factory.
Problem is that the trains are coming from southeast, so the lower right station is closest to them, they tend to unload there, and unless the station has a train sitting in it unloading when next one arrives in the waiting queue, they won't even consider going to any of the other 3 stations if there is a request for material from the bottom right one.
If it wasn't for the fact that the trains are always targeting the closest enabled station to them, i wouldn't even think about station groups and stuff like that. But i can't really control where the train goes once it starts moving, i can't make it skip a station, and while i could force the trains to follow round robin unload on these 4 stations using combinator black magic, the same solution would be next to impossible to use on segments that use more than just single material. Because i can't see what the train is carrying, i can only tell that there is one there. Unless i want to add yet another station into the loop.
At this base scale (about 21 square kilometers) the situation is quickly getting worse.
Somehow i don't think directing few trains should be so complicated, the train groups could solve it. Even if we can't pick delivery to specific stations within the group, a round robin delivery within the group would be 1000% better than what it is right now.
I think i understand the concern of the devs over feature creep. Doing one thing right is better than doing five things half assed. At the same time, as a player/base builder, i have to consider the same feature creep, making complicated things out of combinators and having to come back weeks later hunting for rogue signal really sucks. I'm not opposed to mods either, i use them right now, but i'm not sure if this could be implemented as a mod.