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I'm just starting to dip my toes in circuit network stuff and I have a very basic question about train circuit wait conditions. I'm working on the train for my uranium outpost and I have it setup where it has a fluid wagon that tops off the outpost with sulfuric acid and cargo wagons that pick up the uranium ore. I ran into the issue where I can't use the "Full cargo" and "Empty cargo" conditions because we're emptying the acid where we're filling the ore and vice versa. So far I've been using the "inactivity" conditions instead and this works okay but is a little flaky, I think because whenever a tiny amount of sulfuric acid is pumped out periodically it trips the activity timer. I want to change it to use a circuit condition based on the uranium ore count.

The first thing I tried was just setting the train at the uranium drop-off station to the condition "uranium ore = 0." That didn't do anything, which I suppose makes sense because the train stop reports that it's not connected to any circuit networks. So I got it to work by just connecting the stop to an arithmetic combinator that adds zero to the uranium count and outputs that back to the train stop (which have to be on different colored wires to avoid a feedback loop, which tripped me up initially). This works but it feels like overkill and kind of silly to connect the stop to itself. From searching around I couldn't find anyone talking about this which makes me think I'm missing something basic. I kind of expected the circuit condition to "just work" on the train cargo without me having to connect anything.

When the train stop is not connected to any circuit network, the wait-condition "circuit condition" should use the contents of the train.
I use the same setup and Ore=0 on unloading and Ore > x on loading side works fine.

Use Item-count instead of Circuit-condition. Sorry for bad advise.

Weird - it doesn't seem to be working for me. Here's my train at the station with that condition setup and empty (uranium) cargo and it's stopped... I'm playing vanilla on the latest stable patch.

Circuit condition means that you are connecting the train stop to the circuit network to determine when to send the train off.

Instead of doing this, what you are looking to do is use Item Count.

I usually use the circuit network to enable/disable a train stop. I use the train’s contents, or inactivity (depending on the train) to determine when to leave the stop for the next one.

For a uranium mine, I’d have two (or more, for other things) train stops : Uranium Mine, and Uranium Mine Sulfuric Acid.

Uranium Mine would have 6 loading chests per cargo wagon. All of the chests are wired together to the input of an Arithmetic Combinator. The AC divides the Uranium Ore signal by the total amount the train can hold, and outputs it to the same signal (Uranium / capacity, output Uranium). The output of the AC is then sent to the train stop. Enable when Uranium > 1. (If you want to be fancy, or potentially have multiple trains looking to carry uranium ore, set the stop limit to the same Uranium signal).

Uranium Mine Sulfuric Acid would cater to a train of a single locomotive and a fluid wagon. That fluid wagon would empty into a storage tank, which is then pumped into another storage tank before going to the miners. Wire the first storage tank to the train stop, set to enable when Sulfuric Acid = 0.

If you want to use a combined train, then use Item Count for both stops (Item Uranium = max capacity, or Item Uranium = 0) and let the sulfuric acid top off top off the tank at the mine every time the train visits.

Ah I don't know how I missed the "Item count" condition in there. Thank you!