press E on the square thing on the cart hope that helps

Originally posted by poopyfart69:

press E on the square thing on the cart hope that helps

You'll also might need to use no-clip and make sure that the couplings are close enough together.
(Tip: Any "rolling stock" that has been spawned in after the tender(regardless of whether it has been manually or not) will automatically couple up when you back the tender into it.
On the other hand: You'll still need to couple the tender to the engine manually)