This station provides 12 compressed blue belts of output from a 1-4 train setup due to limiting the speed of the stack inserter. Additional 12to12 balancer to balance wagons with each other. Initial idea by u/Piexes, see https://www.reddit.com/r/factorio/comments/cs3k5h/using_circuits_to_compress_a_blue_belt_with_4/
Explanation:
With the change to belt capacity in 0.17, it's now easier to get full compression using inserters.
A blue belt carries 45 items/second, or 3/8 items/tick on each side.
A stack inserter takes 26 ticks to complete a full swing. When dropping onto a blue belt, it takes an additional (12 items) x (8/3 ticks/item) = 32 ticks to completely empty its hand. So a fully-upgraded stack inserter takes 26 + 32 = 58 ticks to load a stack onto a blue belt at max speed.
A pair of stack inserters can load a blue belt at 24 items per 58 ticks. By slowing down each inserter to only swing once every 64 ticks, we get 24/64 = 3/8 items/tick, exactly the capacity of one side of a blue belt.
All that's left is offsetting the timing of the second inserter so that it doesn't get thrown off by trying to load onto a belt that already has items on it. A blue belt carries items at a speed of ~10.67 ticks/tile, with an addition of 3 tiles of length occupied by the 12 items from the first inserter. So if the inserters are separated by 9 tiles between them, we need to offset the second inserter by (3 + 9 tiles) x (10.6 tick/tile) = ~128 ticks. This is exactly two cycles of our 64 ticks, meaning both inserters need to operate on the same tick.