What is this?
A stacker with a quirk! - It can stack short trains and long trains simultaneously!
A long train is defined as "a train twice as long as a short train"
Take a closer look at the blueprint picture (<-----), you will notice some things:
- some trains are long (2-4-2)
- some trains are short! (1-2-1)
- on the second lane from the left, there is a green short train waiting at the bottom of the stacker
- another blue short train is entering the same lane! (rather than taking the empty lane)
This is mostly what the stacker does.
If the incoming train is short, it will be routed to a lane with another short train in it. (if there is one)
If the incoming train is long, it will be routed to an empty lane only. (or wait for a lane to clear)
The design is scalable. You can have as many lanes as you (might reasonably) want and have them as long as you want! This blueprint book has multiple components that will let you create your own stacker.
Video overview/instructions can be found here: https://www.youtube.com/watch?v=x6-P74xYvYg
The only issue
This doesn't happen all the time, and a way to prevent this is to make sure that your trains can get out of the stacker at a reliable rate, but you should know this stuff nonetheless.
In a very specific case, a train might get stuck at the stacker entrance:
- If a short train is entering the stacker, and
- if the only possible path for it is behind a second short train, and
- if the second short train has been waiting in the stacker for more than 8 minutes...
...the incoming short train will not go where it should. It will stop at the stacker entrance and be "stuck" there for a while.
Sometimes it could get "unstuck" by itself, but in the worst case, it will be stuck there until the stacker gets completely empty! And that usually causes a major clogging of the rail network!
This happens because of how the pathfinder penatly works. There is a thread on Factorio forums with additional info about this issue.