Can't separate minecarts and chests??

cheers mate ill give that a whirl

The minecart travels very different without you in it. I have found that you basically need to double or even triple the powered tracks.

If you have powered rails, the minecart with chest is a good way to to put what you have mined in the chest and send it to the cave/mine opening, house, central destination or wherever you want without having to stop and go back or mine your way out of your
current spot.

Minecart with furnace acts as a steam engine, so you don't need powered rails, though they need a fuel source. Unlike powered rails, they have to be mined and replaced at the back of the "train" to go in the other direction, unless you made a circular station
or redstone device to stop it and/or reverse it.

One is to fill with items you can't / don't want to carry. The other is to push it to its destination. If you'd like you can put your mine cart in the middle and go for the ride.

They are for convenience, so you don't have to run all the way back to where you originally put your furnace/chest. It is true that you cannot ride the cart with these items but you can push the cart in the direction you want it to go.