Need some help understanding train pathing

[Zuckerbirke]

Hi Factorio Community!

I am currently trying to build a Train Unloading Station with a Stacker(?). I thought that I knew enough about train signals and pathing to build it, but it seems that I don't.

For my test setup I had two unloading station and two stacker lines. I began with three trains, and everything was working. Then I added one additional train to see if still works, but it sort of doesn't.

Here is the starting point. One train at Station 1 (the lower one) and two in the stacker, wanting to go to Station 1. The train "Balthazar" is waiting before the stacker wanting to go to Station 2 (the upper one).

Starting point

Screenshot_20260304_232409.png (6.78 MiB) Viewed 153 times

Now the locomotive in Station 1 went away, and the upper train in the stacker moved to Station 1. Now should be an open path for the waiting train "Balthazar" to go to Station 2, but it doesn't take it. It still wants to go through the lower part of the stacker instead of the open one.

"Balthazar" not wanting to use the free path

Screenshot_20260304_232418.png (6.61 MiB) Viewed 153 times

Is it because it already calculated the path and doesn't want to change it? I think the obvious solution would be to just make sure, that the <Unloading Lines> + <Stacker Lines> + 1 >= Train count. But I still want to know if my assumption is correct, or if I messed up something in my design.

[Loewchen]

Is it because it already calculated the path and doesn't want to change it?

Yes. Path calculations are expensive, so recalculations are only done in certain situations. Alternatively to extending the stacker you can make the bays in your stacker stations themselves and add their name in the trains schedules before the main station, this also gives you better input control if the main station gets full.