a graceful optional station
Here is a way to make a redstone line along your minecart track so that you can choose to pass all stations:
The redstone line runs parallel along the minecart track. I will try to explain how to do it the best I can...
First at the terminus (butter minecart station), provide two buttons. The left one (right to direction of travel) stops all stations, while the right one (left to direction of travel) is non-stop. (The repeater bit is actually not absolutely needed here, but I will explain what it is below.)
The redstone wire can go up to 15 blocks starting with full power, so put a repeater (setting to 1 tick is enough) every 16 blocks to extend it. However I suspect it won't go beyond unloaded chunks? Even if it does, there will not be an easy way to reset it.
Therefore, I'll add a latch every few chunks (not tested but I think it can be more than 8). (And here's the main point of this answer.) The latch looks like this (Edit: simplified version at bottom):
And the underneath revealed:
Basically, the latch "locks" the redstone power state, so that it can keep its state even when the source chunk is unloaded. The detector rail sends a redstone signal to reset the latch.
For the station that you want to pass, just use the redstone line to power the powered rails. If the line is not powered you will be stopped at the station. Everything else is exactly the same.
If you want to be able to do this for the reversed trip, just copy the setup in the other side. Due to the redstone line being single-direction, you cannot simply reuse it.
Also, you will not be able to change from "non-stop" to "stop all stations" without taking the whole trip (as I said, the detector rails reset the latches). But the opposite is possible, i.e. you can change to non-stop in the middle of the trip by putting a button (or a trapped chest if you want a bigger target) and use it to power the redstone line.
Tested on Minecraft 1.5 with the station beyond 40 chunks (640 blocks), not long but definitely have exceeded the chunk loading distance).
Edit: (x2) Didn't realize that it can further be simplified a bit like this until I've posted the answer:
The best/simplest way to do this (and something I've used before) is to make the carts simply pause for a few seconds at each station. If you want to stop at that station, you climb out and walk away. The cart then continues on and gets dumped onto a side track. Otherwise you stay in the cart and it'll take off again shortly. It's not quite as elegant as what you wanted, but it's quite a bit better than jumping out of a running cart, and easier than hitting a button at every stop.
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:
- The incoming track needs to be straight for some distance before, so that you can easily position your cursor to hit the cart. You probably also want to have the chest a little bit earlier along the track.
- There must be exactly 1 redstone dust between the chest and torch. The signal is too weak to travel farther, and a block beside the chest won't work at all. Don't know about using a redstone repeater.
