For automatic item-sorting rail systems it is usually much easier to destroy a used minecart and re-deploy it at the start than it is to add another rail system that returns it to home.

OR, add the ability to dispence a chest onto a minecart. that would achieve the same thing and be kind-of cool.

The second thing already exists, that should solve any thing you need to do. You can dispense hoppers, chests and tnt into minecarts.

oh. thanks.