Programming my filter loaders to turn on and off based on how many wagons a docked train has

[karmic salmon]

Hey engineers,

I've finally entered the mega base train portion of my IR3 playthrough. Now I know a solution to my problem is simply having all the same train lengths but I'm still transitioning so plenty of different wagon sized trains. Is it possible to configure a stop to turn loaders or insterters on or off depending on the number of wagons of the docked train?

In my screenshot I'm essentially just trying to turn just a few lanes on when a single wagon shows up. This warehouse has several different ingots and right now I have it able to turn the filters on depending on what the train is asking for but not able to figure out if I can read the # of wagons. I ran into an issue previously when I just equally loaded all lanes because the trains would show up demanding goods but the single wagon box would be empty while the others weren't, causing it to stall. I know I can begin loading from the back and have it trickle all the way to the front, ensuring the front is always loaded first which I will evetnaully do if I can't figure this out hehe.

[steady breach]

Base game this is not trivially possible. Some mega brain circuit could be used to map train length to train ID but you probably do not want that.

With LTN and Cybersyn there is output logic you can read for where coaches exist in a train that was dispatched to the station.

[karmic salmon]

Thanks I'm using Cybersyn so I'll take a look on the mod page

[round wolf]

can't you just place a few rail signals to detect wagons?
tie it to a train being parked and it should be relatively simple to to get the train length

[steady breach]

In that case you use the cybersyn combinator in wagon mode. You place in near where each wagon will stop. It then reads out how much the wagon wants, if there is a wagon present. It should not have a request when no wagon is present, in theory at least.

[steady breach]

Assuming it is not detecting a second train behind.

[vale scroll]

You'd be able to use chain signals for this, trains won't enter the station unless they can stop at the train stop itself if you place a chain signal behind every wagon and use that to measure the length of the train

[indigo heart]

in addition, the mod https://mods.factorio.com/mod/Inventory Sensor can detect whether there's a wagon in front of it, and whether it's a cargo wagon, fluid wagon, arti wagon, or locomotive.

[dusky tide]

If you don't have trains with same lengts just use signals to calc train length