Now we need to focus on the tracks that lead into and out of our station.
Before starting any work my Arrival track is only connected to the station I built earlier and the Departure track is only connected to spawn.
Lets look at the critical area a little closer...
In this image we can see the current state of the tracks. The problems we need to solve are:
Both tracks leading in to the station need to always use the Arrival track.
We need a way of choosing which track we will use when departing the station.
A word of warning, I screw this up pretty badly and then have to patch what I did.
So this revision brings both tracks automatically into the Arrival track so my incorrect thinking is that I just need the ability to switch the new junction (the junction near where the hand holding the track piece is). How wrong I am!
I power the junction with a redstone torch and even now I still don't see the problem. I'm now considering the departing carts in my mind - which this setup works for, but I stopped thinking about the arriving carts. Can you spot the problems yet?
I was aware that carts arriving from my original station would be automatically diverted on to the spawn track and would miss my station completely so I thought this was the answer.
Carts arriving from the right now are diverted, and I can switch departing carts to either track.
Have you spotted the flaw in the logic yet? I hadn't so I blissfully continue on.
Using F3 over the junction I get my coordinates.
Back at the station I place a lever, turn it on, and start laying wire toward my junction. This will need to be activated from below so out comes the pick.
The wire affects a redstone torch under the junction.
Now to return to the surface and test the linkages...
Lever down will divert departing carts to spawn...
And lever up will divert carts to my other station.
At this point I realise that I had completely disregarded carts arriving from spawn. Crap!!! This isn't going to work!
I try to think through various options. I was considering the so-called south-west rule but my track needed to travel east. I was considering building a track bridge to divert traffic.
In the end the solution was fairly simple...
Where my old junction was is now an unchangeable special cross-road. All traffic coming from spawn travels straight across and onto the Arrivals track. All traffic from my other station travels around the curve and onto the Arrivals track.
By placing my junction switch at the end of the slope then departing carts can be diverted either to spawn or to my old station.
A quick change to the circuit under ground...
And then test the results...
Lever up diverts departing carts to spawn.
Lever down and the carts will travel straight across the junction and then curve onto to the other track leading to my other station.
So in the end the solution was fairly simple, but it did take a bit of trial and error to arrive at the answer.