There are quite a lot of mods that add new chests and other storages to Minecraft, mods like Iron Chests, Factorization (barrels), or JABBA.

Would be nice if you could craft those into chest/barrel carts.

I would like to mention that I - at least - consider this issue resolved as of 9.8.0.0

The cargo cart is almost exactly what I wanted when I posted this request.

For what it's worth, Factoritzation added Barrel Carts months ago.

There's a bunch of new mods coming with that sort of thing new types of carts , i agree with vexatos.

No need for bigger carts there is a seperate mod for that

Curseforge

http://minecraft.curseforge.com/mc-mods/224989-iron-chest-minecarts

Github

https://github.com/ganymedes01/IronChestMinecarts

I do not take credit for this mod all credit goes to its creator

Considering this request an extension of the filtered minecarts. The ability to make minecart that are visually different with either color or emblems would enhance systems we build with loadwra greatly.

Sure naming is one way but visual differnce would make it possible to make visually stunning trains.

Sorry, I won't be adding any "increased storage" carts to Railcraft. You're supposed to use trains. I'd like to add some storage carts with filters though.

The main reason I asked, was for "barrel carts", or similar carts that would only store one/a few types of resources.

So I would say that filtered storage carts would fill the spirit of the request.
Would be nice if there could also be a way to detect those filters from item loaders/unloaders too.

Filters carts would essentially make them function as barrels with lower capacity.

As for visually different - would http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2091897-cartlivery cover your needs? (I don't know the mod, just found it on the forums)