Cart linking is a system added by Railcraft that allows carts to link together to become a train. This should work with any cart from any mod, all that is needed are Minecarts and a Crowbar. Note that the Crowbar will lose one point of durability per use.
Position two Minecarts on the track next to one another.
Shift right-click one of the carts to be link. The string "Link Started" will appear in the chat box.
Shift right-click the second cart to link. "Link Created" will appear in the chat box if successful. If "Link Failed" appears, move the carts closer together and try again.