Wow, this certainly makes spiders a major concern again. I just met my end after a zombie lured me out of my base waving a bit of leather. His spider buddy came at me from behind and stuck me to the spot. The zombie finished me off and let out a very punctuating burp to rub salt in what was left of ...

I can't figure how you can tell the cart is getting filled, rather than items just dropping out of the hopper.

Would a detector block detect just the minecart, or does it pick up the rail too? If it only detects the minecart then you could use that to control the mechanical power to the hopper.

IIRC a powered rail won't move a cart unless the cart is already in motion or there is a solid block behind the cart, so you should be able to use a block on a sticky piston to trigger the rail. Battosay had a great video of a minecart transport system that triggered the rails like this, although I'...