Railcraft started off as a simple minecart mod, but it has since become much more than that.
It features a fully fleshed out tech tree, with great effort gone into providing a balanced gameplay.
Railcraft was also one of the pioneer mods when it comes to Multi-Block structures. It features a whole slew of these structures, including the mighty Steam Boiler and the stunningly massive Iron Tanks.
For more detailed information regarding the mod, you should visit the wiki here.