I'm thinking you are trying to use trains to move items around that could be done as well with paths( or chest highways). Expand on that learning moment you had to pipe in the wood to the trains and keep trains as the long haullers they work best being. Also remember you can unsushi a path or chest highway many ways so it doesn't need to be 1 route per item. A path/chest highway loooping between train carriage inputs and outputs can let the question of which items are on the train be mute as it will be picked out from the sushi belt mid-loop between the stations when needed.
So, it is probably more than you want to do, but you could create your own "sensor" but having one stumpy spark pick the first item off a carriage, then using the path item filters walk across different weight pads, which in turn would trigger the correct haulers to start unloading... That seems like a lot, so it might also just be better to wait for a sensor, if they exist. 😅
So using the 3 lines per carriage, 1 would be a "sensor" unloading spark to then trigger on of the other 2 paths, with an 2 paths with weight pads in them behind item filtered gates, to open which hauly spark to unload the train correctly? With this you could also reverse the idea a bit and always unload the train and send the items back up the other side if it's not the items needed, setting the train to depart only when fully loaded( or use logic from the corner of the station to stop it from going until reloaded.
I'm thinking you are trying to use trains to move items around that could be done as well with paths( or chest highways). Expand on that learning moment you had to pipe in the wood to the trains and keep trains as the long haullers they work best being.
Also remember you can unsushi a path or chest highway many ways so it doesn't need to be 1 route per item. A path/chest highway loooping between train carriage inputs and outputs can let the question of which items are on the train be mute as it will be picked out from the sushi belt mid-loop between the stations when needed.
So, it is probably more than you want to do, but you could create your own "sensor" but having one stumpy spark pick the first item off a carriage, then using the path item filters walk across different weight pads, which in turn would trigger the correct haulers to start unloading... That seems like a lot, so it might also just be better to wait for a sensor, if they exist. 😅
So using the 3 lines per carriage, 1 would be a "sensor" unloading spark to then trigger on of the other 2 paths, with an 2 paths with weight pads in them behind item filtered gates, to open which hauly spark to unload the train correctly? With this you could also reverse the idea a bit and always unload the train and send the items back up the other side if it's not the items needed, setting the train to depart only when fully loaded( or use logic from the corner of the station to stop it from going until reloaded.