Startlingly simple: Cart dispenser will spit out a cart on a redstone signal, or absorb one if one is present. Place a detector track on top of the dispenser and incoming carts will be sucked in. You can then pipe them into a second dispenser, and when you press a button to provide a signal, or step on a pressure plate, it will spit a cart out on top and you'll be ready to ride. Leave a blank track on top and then you can use W to push onto a booster (powered) rail, and speed away, or have a locking track from railcraft that will give you a shove with redstone. If you want automated leaving, you have a pressure plate/trip wire you cross, cart is ejected and sent off, and crosses a boarding track that automatically sucks you aboard.
One station at either end, and poof, cart garages with on-demand carts.
Of course rhn's works as well. For whatever reason couldn't post yesterday, or I'd have done it then.