This stream runs about 38 minutes and opens on a card reading "Stream Archive" and "Train Length Detection", with no episode number, so the number here comes from the playlist. It happens on the Hypermine Vanilla server under Basket Station, apart from a minute in Basket's creative world to check a design. Scoti_Garbidis joins partway through: "Scoti_Garbidis joined the game" shows in the in-game chat, followed by "<Scoti_Garbidis> o/".
Counting the cars
The throttle is already in place: chest carts with items come out of it slowly, one at a time. The next job is to count how many there are and send them the right way. Basket loops the line around and up a little to get some room, and runs powered rail across it.
Then comes a quick trip to the creative world as a refresher, since "it's been quite a week or so since I designed this." The test build used a tripwire hook, but with the cars already slowed down, a detector rail will do. A lone car turns off to one side. Two or more go straight, so the whole train stays together.
Swords in a dropper
Back on the server, Basket crafts detector rails and droppers and builds the counter. Each car that passes the first detector rail drops a wooden sword into a dropper, and a comparator reads how many are there. The line splits in two after that, and a detector rail on each side moves a sword back as a car leaves. A redstone torch flips the track when the count is high enough. Basket crafts 11 swords, nine plus a reserve, and loads them into the hopper.
A single chest cart with some cobblestone in it goes first. It waits, zooms over, counts as one, and lands in the right spot.
The four-car test
Next, a test train: four chest carts on the caution floor, each holding an item, launched with the button. All four go straight, so the count works. But two of them land with the empty carts.
The problem is the sorter from before, the part that checks whether a chest cart has items in it. It grabs an item from every cart but only sends about half of them down the loaded line. "That's actually what I'm trying to rely on my other station," Basket says. One fix is a throttle at the small rail station too, but there's not much room there. A second run of four does the same thing: every other car.
Same item or not
A viewer suggests sending just two. "Okay, good point." Basket builds a closer copy downstairs, with a button and levers, so the carts come through at the same speed as they do up top. With the same item in both, the second cart goes the wrong way, as expected. Basket's guess is that matching items stack inside the sorter, so the test goes again with redstone dust in one and iron in the other. Same result. "So it doesn't matter if they are the same item."
Still to fix
What does work is the length check. Basket isn't fully sure of that either: after all those tests only one cart sits in the single-car spot. And the counter is off by one. It holds one more sword than it should, so it thinks a cart is still somewhere between the detectors. Both problems wait for next time. The credits run to the end of the video with no gameplay after them.