The Tank Cart is from the RailCraft mod and is used to store and transport Water, Lava or any BuildCraft liquid such as Oil or Fuel. It is filled/emptied at a Liquid Loader/Unloader. It's capacity is half of a BuildCraft Tank, meaning 8 buckets. Tank Carts are affected like regular Minecarts. They take approximately 60 seconds to completely fill/empty using Liquid Loaders.
Tank Carts are an un-buggy alternative to Teleportation Pipes. They are a cheap way to transport liquids such as Oil and Fuel.
There is also a glitch with some computers where you can duplicate items. Due to this fact, they are banned on many servers.
Tank Cart (Creosote Oil)
Tank Cart (Water)
Tank Cart (Lava)
If any stackable items are placed in the bottom right corner of the inventory, excluding the hotbar, then shiftclicking will give a full stack of the item. This works with all stackable items from EE and IC2.