I can't find any guides on how "filter rail"s are supposed to work... hitting with the wrench doesn't do anything... I can open the gui of the track but... no matter what I do, minecarts go straight across the filter rail.

You shift right click the rail with a minecart tinkerer. Then you'll get an inventory where you place a filter item. Left clicking any minecart with a minecart tinkerer will open a GUI where you can add a filter item to that cart. Any minecart which has the same filter item set as the filter rail will then move around the corner. Any other minecart will go straight.

Ahhh... Thanks... Didn't see there's a way to open a GUI on minecarts as well... does it work with all kinds of carts (i.e. non-utilitix, non-minecraft carts... like create or any other ones)?

Ok... Reading the "U"-Info carefully would have helped... Sorry for being stupid.

It should work on all types of minecarts, even carts from other mods.