Before the 1.21.20 update, it used to be possible to have detector rail above a hopper and using a comparator, detect when a minecart was empty as a result of sitting on the hopper so additional actions could be taken... like moving the minecart along. This no longer works. It appears that the detector rail now locks the hopper as soon as the minecart land on it meaning the hopper won't empty the minecart meaning the minecart will sit there forever.