Factorio Forums

So, in the screenshot below you can see two trains on either side of a set of signals. For whatever reason the signals are showing green, and allowing the trains to run causes a collision.
I can fix it by putting signals at the end of the bottom loop, near the intersection:
For the life of me I cannot understand the logic behind this? Can someone explain this to me?

Do you have a circular system?

I think you have a circle of tracks with just one signal on it. Since the signal loops back to itself, the signal stays green (ie you can't divide a circle with one cut). By adding the other signal(s), you are dividing the circle into 2 segments that can be blocked individually.

The solution would be to add signals to the rear of the parked train, as well as at the branch, so the next train can proceed to the end of the line.

As a rule of thumb, I try to put signals at every decision point.

Apart from the loop I don't have a circular system, no. The two tracks to the left head off to two separate outposts. The whole system is in the last video of my Dytech series(if you need to see it), as well as the trains crashing into each other.

I have the solution already, I just wanted to know about this double green situation when the train is clearly there.

The tracks to the right connect into a circle and a single set of signals on a circle can't break it into 2 segments.

Oh wow, so signals are a lot more complicated than I initially thought. But that makes a lot of sense, thankyou. I guess that clears it up.