if you put a minecart hopper on a detector rail then you can use have a comparator to see how many items are in it but any minecart hopper on a detector rail blocks it from putting items out due to the fact that the hopper minecart powers itself but that means that it is not possible to compare and move items at the same time which should not be a problem so yeah
Comments 3
This is a change request, everything is working as expected. The detector rail is not powering the minecart, but hard-powering the hopper beneath the rail.
This is a problem because it prevents the creation of item filters that sucks items through blocks.
If you want a hidden item filter under the floor, you have to use a minecart hopper, but you can't have a hopper below pulling items AND mesuring the minecart with a comparator for the filter setup.
I believe we should be able to do that kind of setup.