If you have a Minecart with a Hopper sitting under a Chest and on top of an Activator Rail, it only pulls out if the rail is ON, not OFF. It more sense if it pulled out when the Activator Rail was off, not on, seeing as the Activator Rail essentially powers the Minecart above it when on, and Hoppers only pull out when not receiving a redstone signal. If this is intended, I appologize, but I just think it'd make more sense the other way.

tl;dr: Hoppers don't pull out when on, Minecarts with Hoppers, if they're on Activator Rails, only pull out when on.