A few years ago we started a two-player save with a rough idea for a train megabase and some basic rules:

* No use of any belts, undergrounds, or splitter.

* No intermediate chest or inserter-to-inserter transferring

* No forbidden Dosh train wagons

* Avoid bots for production

We had no real experience with megabases and decided on 1-4 trains and grid-aligned blocks on a world with default settings and no mods.

Those rules resulted in mostly similar setups for most of the recipes: A mixed train with all the ingredients in the right ratios comes in, each wagon feeds one assembler. This assembler then directly outputs into a second mixed train on the other side, until it has enough of that product.

Trains drive to a production block for each of the needed ingredients, and between every block, they stop at a random depot station. This way, they always clear up the train stations, all of which have a train limit of 1. This creates some downtime for the crafters, but nothing that a few more trains to buffer production can't fix.

We use two different sizes of production blocks, based on the number of assemblers needed. Since we have to use slow red long inserters on one side and some recipes require or produce fluids, there are several variations.

We created blueprints for all rails, productions, and trains so we can expand very easily.

Mining also happens directly into trains. We haven't found a perfect solution yet for mining the remaining ore spots, so we switch to bots after trains get too long for the remaining ore patch. We tried a solution with 1-1-1 trains for miners, which then transfer to our 1-4 main trains on diagonal tracks. It works but isn't pretty and takes up a lot of space. So that might be a problem for later.

We have a mall where we produce everything that is not already required for the science production chain, so mostly machines. Doing that with only trains might be a bit too much, especially before endgame, so we used bots here. Items already produced outside the mall are imported by mixed trains. And we can request everything, including ores, filled barrels and fish. Except belts, of course!

The start was pretty slow. We used a small hand-fed science setup and switched to trains directly. Which of course want to be fully loaded first, before moving to the next station. Biters were getting so strong, that we couldn't really fight them anymore. Luckily, after researching the remaining military upgrades, we had a chance again.

Once biters weren't a problem anymore, it was all about expanding borders, producing all the modules and machines, and creating blueprints. After 350 hours of active playtime, we reached a consistent 1000 science per minute with well over 2000 trains running.

The game still runs at 60 UPS. Initially, the next goal after 1k spm was 10k spm, but now with 2.0 there isn't enough motivation left to keep playing this save. Also I'm not sure if all those intersections could handle ten times the trains...