Wikia

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 have the same properties as regular carts do, without the function to ride in them. They take approximately 60 seconds to completely fill/empty using Liquid Loaders.

Due to the addition of teleport pipes, however, there is little need for them unless a player already has an existing rail network. This is a cheap way to transport liquids such as Oil and Fuel.