Why?
-Trains need filtering. That is why the Cargo Wagons have filters. If you are building a rail world where you probably have millions of trains running, the Train identification system in the circuit network just doesn't cut it. You need a general way to tell the difference between an Oil transporter and a Steam supplier train. Especially when they often come to the same stop.-Pumps are the liquid equivalent of inserters. (Right?) I would rather ask why not? I have tried replicating a filter mechanism with two pumps, three storage tanks, and some circuitry, but I can't find a way to detect liquid that is less than 1 unit in my tanks. My tank keeps showing me less than 0.5 units of water and both pumps are off because circuitry can't detect it.
-Having theese features will finally allow for generic or multipurpose stations that can supply and unload any train.
What?
Filtering on Fluid Wagons. Setting the filters will make pumps that load the wagons only accept the given liquid atop of their own filters.Filters on Pumps should work like filter inserters. You set a liquid, and that and only that type of liquid can pass through the pump. Other types of liquid simply don't enter the pump. It would also mean it should be possible to set the filters through the circuit network. The same as on inserters.
Extra: Pumps should be able to report the train wagon filters to the circuit network.