The tank cart uses Item Loaders to transport liquids over rails.
Note that if placed below an empty Liquid Loader it will not begin filling even if the Liquid Loader receives liquid. The Tank Cart has to be moved away from the Loader and back under it before it starts filling.
Railcraft's tank cart is made with an iron tank gauge and a minecart.
It is possible to craft the Tank Cart with any liquid container containing liquid to get a filtered Tank Cart for that Liquid. However it is also possible to simply place a Tank Cart, open the GUI and click with the container in the filter slot, then break the Tank Cart again, to get the same filtered cart without the extra cost of a liquid container.