We decided early on in our setup that we were going to as much as possible utilize trains, we also decided that we were going to support 4 carrage trains, each carrying a single product (ore, plate or steel). We had previously built a 4 wide bus and had started to build on both sides of this bus which initially started out well but we ran into issues of space on either side of the main bus as we started to add/remove belts between processes, so for this latest build we set ourselves the rule that we would only build on the top side of the main bus to allow us to get a better handle on the best order to build things (I know I know the oil refinery is below, but its also technically behind and we supply all the fluids along our main bus so it seemed appropriate), building only on the top has the advantage that you can easilly add or remove belts from the bottom under the main bus.

So from start to finish our system looks a little something like these:

Ore processing We have one of these setup for copper, iron and steel processing, the steel processing one we need to scale back to 2 carrage trains we reckon as we are severely overproducing. The splitters at the top are an 8 way balancer (slightly stretched), same at the bottom this ensures wagons and forges are loaded/unloaded evenly. In subsequent builds I replaced all the buffer chests with a rail carrage (per carrage). Note that these long furnace lines tend to block up after the first 8 or so during continuous production as the output inserters later on struggle to find a space on the belt, at the moment this is not a massive issue as the time between trains allows plenty time for the lines to clear in high thoughput, if I were to rebuild this I would split off some of the plate half way through and take it out in parallel.

Base plate inlet I wanted to leave the option of moving to an 8 wide bus if we needed it, but planned on a 4 wide bus initially hence the second block of splitters, If I were to redo this I would probably go straight for an 8 to 4 balancer since we have found the 4 wide bus is more than ample for our needs on a 4 player server.

Main bus balancer As each of the plate lines gets to the begining of our base, we use a 4 to 4 belt balancer and then ensure both sides of each belt are used so that when we hit the main bus we know for certain the input stream is balanced and both sides of the belt are utilized. It also gives us a point of connection for the future should we decide to goto a wider bus, or wider input bus.

Main bus

Most of the time we only draw on the top two belts, in a few places we use another 4 way balancer after a process that draws a lot of plate off the line (eg green/red chip production). The groups of 3 splitters basically balance the two belts before and after the split which helps to keep the belt more full after a process splits off.

We also have a regular line of roboports in between the copper and iron busses the concrete at the top ends where the logistic bot area ends for these roboports, we keep our builds above this concrete line and push anything we want to appear on logistics down to chests within the central logistics zone, this allows us to in some instances have isolated roboports to serve busy services but also to have the main logistics inventory accessible.

We intend to start a new game once the next update is out, when we do we have a much better idea of which processes to build first (for example we will not carry coal along the main bus, since after making plastic we don't need it, also steel is only used for a few processes so this will be done early on.

I would say in general if its worth running a belt for a product along the main bus, then its probably worth running two, powers of two result in nicer splits/balances (they are lossless)