Games

So, I have 2 train routes for exports, highlighted for visibility:

• A cement train that runs between the red line (at the end of which is a loading station for cement) and the customs house at the far end of the green line
• A gravel train that runs from the aggregate loading station at the end of the blue line to the same customs house

Here's the behavior I want:

• When returning from the green route, a train has right-of-way to return to its loading station
• If a train arrives at the intersection from the red or blue route, it should proceed only if there is no train on the green line; otherwise, it should wait until the green line is clear

I'm not entirely sure how semaphores work and no combination of them seems to do the trick. I get trains unable to return from the customs house, trains colliding, or trains forever deadlocked at the junction. I've watched/read like 3 different tutorials and I'm still not sure what I'm supposed to do.