Placing a hopper underneath a chest minecart allows the hopper to remove items from a chest and either store them in its' 5 slot inventory, OR place into a block adjacent that it is connected to.
Also, detector rails with a chest minecart on top and a comparator next to the detector rail will supposedly do the same functionality as when a comparator is placed next to a regular storage block or block with an inventory and output a signal strength according to how full the storage block is.
Hoppers when having a redstone current applied will no longer receive or dispense items.
Having said all of this, I will now say that minecarts traveling over detector rails will cause the detector rail to power nearly blocks, including hoppers, which causes them to not receive items from said minecart.
I would like to suggest that detector rails do not power hoppers. (Bold, to easily see what the suggestion is)
Why? Because if you wish to set up an item transportation system using chest minecarts, the most efficient way would be to use a comparator determining how full the chest in the minecart is. However, in order for the comparator to be compatible, the minecart must be on top of a detector rail. Because of the current functionality, you cannot have a system that will stop a chest minecart on top of a hopper and have the hopper unload items, and once the minecart is empty (or reaches whatever point you choose) have logic gates and other redstone mechanisms send the minecart somewhere else to get refilled or something else because the hopper underneath the minecart gets deactivated by the only possible block to tell the comparator whether or not there are items in the minecart.
Because of this, there is no easy solution for sending a minecart along a track, and if it is empty let it pass over a hopper, but if it has items stop it there until it is empty, and send it on its' way.
I would like to see this changed so that using a detector rail and comparator can be used in conjunction with a hopper system underneath as well. Or create some other block or functionality that would allow a comparator to determine how full a cart is while it is on a hopper AND let the hopper still unload items from the cart. Currently the only use for comparators and detector rails is sending empty carts one way, and full carts another, (such as to a hopper station) but you would still have to rely on a timer to decide when to send the minecart back along its' way.
I have discovered a workaround to have a hopper unload items from a minecart and send it off from the unloading station once it is empty. It requires placing the detector rail before the rail that the hopper is under and having repeaters to delay it for long enough for the cart to get over the hopper and the hopper to accept at least one item.
At this point a comparator next to the hopper with trigger a pulse extender of some sort to output a signal that the hopper still has items in it. The reason this is needed is so that as the hopper takes out items and places them into a chest the short amount of time that it is empty doesn't give a false empty signal. The hopper then dispenses items somewhere else like a chest or another hopper.
The reason the repeaters are needed is because the detector rail cannot be on the hopper, and so that when the minecart is on top of the track above the hopper it will give the hopper enough time take an item to start the comparator detector without giving a false empty and sending it on its' way.
I still believe creating a new track item that will output the signal to a comparator, or changing how detector rails and hoppers interact would greatly simplify the redstone required to have a minecart chest that is sent on its' way when it is empty without having to use a timer that would be inaccurate.
Any input or suggestions would be appreciated, as well as feedback on if you support this.
Okay, I am now trying to have a Minecart detection system that tells if a Minecart is full before sending it off. I successfully got it to work. Then I tried to have the Minecart fed by a Hopper. Because the Minecart is not an actual block, but the Detector Rail is, placing a Hopper over a Minecart on top of a Detector Rail hooked up to a comparator... It still powers the Hopper meaning it cannot place any items!
This means that the feature of a Chest Minecart on top of a Detector Rail and using a Comparator to determine how full the chest is is essentially useless. All it can be used for is routing a cart with items one direction, and an empty cart another. You cannot use Hoppers at all in this equation which is completely and utterly ridiculous and pointless.
I would again like to suggest that Detector Rails when placed directly next to Hoppers do not deactivate them. However, if you have it power Redstone which then leads to a Hopper, then it will deactivate it.
Either that, or create a new rail block which can be used with a Comparator to determine how full a chest Minecart is, but does not actually provide power to any blocks.
Until then, Comparators and Detector Rails being able to play off of each other are rendered essentially useless.