Minecart with Hopper

A Minecart with Hopper is a special type of minecart functioning in a similar fashion to a Hopper as it goes along its respective rail. It will collect items lying on tracks and from containers above them. It can cause the most lag of all the craftable minecraft entities, as it combines the lag of a hopper (SMP mainly) with the lag of a moving minecart (SMP and single-player). To avoid this lag, it is advisable to disable the cart as often as possible - see below.

Behavior
A Minecart with Hopper will take in items which are lying nearby (range only slightly larger than the cart itself), or inside a container above the track.

It will also pick up items that are lying on a full block above the track. An ordinary hopper will only do this for blocks that are not full height (slabs, soul sand, etc.). This is probably due to the minecart hopper being located slightly higher than a normal hopper.

Unlike an ordinary hopper, items are not automatically transferred from the cart to other containers, but it can be unloaded by a hopper directly underneath the track, just like a Minecart with Chest.

The hopper can be disabled or enabled by using an Activator Rail. When a Minecart with Hopper runs over a powered activator rail, it will stop taking in items; an unpowered activator rail will cause it to resume picking up items. This effect persists even after the cart is no longer on top of the activator rail.

A Redstone Comparator will only work with the Hopper and Chest Minecarts if you place the Minecarts onto a detector rail with a Redstone Comparator next to it.

A Minecart with hopper will travel more than 85 blocks without stopping (as opposed to a normal cart going less than 12 blocks) from a dead stop using a 2 powered track starter even with another cart in front of them. This makes them excellent for moving carts long distances when you have little gold.

However, the distance travelled by a Minecart with Hopper itself is heavily dependent on the hopper's load. Using a 1 powered rail starter track, a minecart with an empty hopper will travel 64 blocks until it stops (as opposed to an empty normal minecart going 8 blocks). The distance travelled diminishes non-linearly with increased hopper load; a minecart with a full hopper will only travel 16 blocks in this setup.