Train Stop Chests Becoming Unbalanced

[LyingDwarf]

Hello hello. I have a train stop design that uses splitters and balancers to ensure that an equal number of items are loaded / unloaded into every chest. The balancer isn't just a belt balancer, it is also a lane balancer to ensure uneven draw from the sides of the belts cannot throw it off. This design used to work flawlessly for me in my past playthroughs. But somewhere along the way through my Space Age playthrough I noticed that the chests were becoming increasingly more and more unbalanced. To be clear, it was for all of my train stops, including ones that did not have to deal with stacked items or different quality levels. I'm wondering if anyone can tell me what changed to cause this self balanced design to no longer work or if anyone else has ran into this? Because I can't seem to find anyone posting about it. And does anyone know a way to fix this design without using circuit logic on every inserter? I know you can use circuit conditions to force them to swing only when their chest is above or below the average contents of all the chests, but I am hoping for a more UPS efficient solution than that.

Here is a screenshot to show what both the requester and provider stops look like. You can ignore the circuits on there, those are there just to auto control the train stop limits based on supply and demand. Oh, and the inserters are a weird color in this one because I'm playing through Bob's overhaul right now.

09-12-2026, 09-21-13.png (1.49 MiB) Viewed 143 times

[Tertius]

The train-to-chest inserters load into 4 chests. To stay 100% balanced, all 4 inserters must either all swing or all not swing. You load iron plates, 4000 items per wagon, with fast inserters (hand size 4) into 4 chests per wagon so each inserter must swing 250 times. This will usually work, so it's not clear what the issue is for you on the screenshot.

However if you use bulk inserters with stack size 12, it's different. 4000/12 is 333 full hands + 4. With 4 chests, its 333/4 = 83 + 1 swings. So every chest except one gets 83 swings, one chest gets 84 swings and one of these gets another swing with just 4 items. I don't remember exactly why, but one and the same chest seems to always get the additional swings so the chests at the unloading station are being filled unbalanced. Game engine update order or something like this. But they're being emptied balanced, so one chest continuously gets more items than the other chests in the long run - it's always the last swing causing this.

The issue will not surface or not that fast if you overfill the chests, i. e. every chest gets completely full before the train gets empty. But if some chests never get full because items are unloaded as fast as being loaded, or you deliberately open the station only when more then a full train load is missing in the buffer chests, unbalancing will increase.

If you want to avoid this, don't change the train limits dynamically - let station chests overfill. Or the hand size multiplied with the amount of chests (per wagon) must be a multiple of the wagon capacity. Or you need an active balancing control that uses the average of the chest content on the wagon-to-chest inserters (not on the chest-to belt inserters!).


Why you don't read about this: People with static train limits are not affected, because they overfill. And often people don't care. They use unbalanced unloading anyway and don't encounter the subtle unbalancing you describe. Many just use more stations than necessary and balancers downstream to cover and hide the unbalancing. It's not efficient, but they don't care. Your issue was brought up years ago (actually, I started some thread and learnt what I wrote above), it is being discussed since the earliest years for sure.

And your setup seems to contain an impossible underground belt that contributes to unbalanced behavior, because just 3 inputs of the balancer are active.
This is underground length 7. Either you use modded underground belts or this belt doesn't transfer any item.

09-12-2026, 23-38-18.png (157.22 KiB) Viewed 67 times

[LyingDwarf]

Oh wow, that's an incredibly thorough breakdown of the math as to why this happens. Thank you for explaining that in such detail, that makes way more sense to me now. I'm still a bit surprised I hadn't noticed this in previous playthroughs, but it's likely it was happening all along and I just never noticed for any of the variety of reasons you listed.

As for the inserters and the seemingly disconnected underground in the picture, that's because this playthrough is using Bob's Mods overhaul, so those are actually bulk inserters and the underground is connected (the undergrounds have much greater lengths in Bobs).

I updated my current blueprints to use additional circuit logic to make each inserter only swing when they are above or below the average of all the chests contents depending on whether it is a requester or provider station.

Thanks again for the breakdown of the math on why this occurs. That honestly makes me a lot less annoyed at the game now that I understand why it is happening.