One interesting property of the new powered rails are that they act as brakes when no power is applied to them. Because of this, you can make extremely simple stations like this one:
▮
▓━─━═━─↔
─ rail ━ booster rail
▮ button ═ detector rail
▓ any block
The minecart rests on the slanted powered rail because it the track acts like a brake. When the button is pressed, the track becomes powered and boosts the cart away.
The sequence of detector, powered, detector track isn't a vital part of the station design, but is instead a 2-way booster. When a cart passes over the detector track, the powered rail becomes powered, making a very compact booster. This booster also helps push the cart up the unpowered slanted rail on arrival, resetting it for its next use.
I've been playing around with ways to make a cart magazine that isn't reliant on the cart boosting glitch. However, it's not as simple as constructing something like this...
...as additional minecarts will fall into the bottom one, melding together, instead of stacking.
YouTube user MinecraftAddict developed a hoizontal-cart-magazine-like concept using powered rails like this.
When the redstone is activated, each powered rail will send the cart to the next spot, while the final one will by launched to wherever you hook it up to. However, simply using a button may power the rails for too long and cause some carts to move two spaces, so you may need to use a monostable circuit.
When the button is not pressed, the redstone torch on the side of the button's block keeps the output torch off. When the button is pressed, the first redstone torch turns off, turning the output torch off. At the same time, the repeaters have a redstone signal propagating through them. When the signal finally reaches the output block, the torch turns off again. By adjusting the delay time of the repeaters, you can adjust how long (short) you want the button press to last.
My suggestion would be to change the normal rail at the bottom of the dip into a powered rail. This also removes the need for two pieces of Redstone dust, as one would power the whole section.
You don't need to use Redstone to bypass that on the way in; you could simply build something like this (it might take you a bit to manipulate the tracks like this):
Best Answer
Here's a quick test implementation of using a Trapped Chest as a large button, as @ratchet freak suggested. Trapped Chests emit a weak redstone signal as long as they are open, stronger the more players have them open.
I wasn't thinking hard enough about how to use it, so this just stops the cart as long as the chest is open, and so is kind of useless — but you can easily change it to send the cart onto a siding for the station. Key points: