"No Path" Error on Train. Resolved, but don't know why.

[besselfunction]

Hi,

Relatively new Factorio player here. I've managed to bootstrap my way through learning most of the game, including trains. I feel like I'm starting to understand signals fairly well, and have some sort of overarching principle to my train route design. I was recently adding a new station near the end of one of my lines, and managed to break everything for this particular stop. I managed to resolve the issue by removing a single signal, highlighted in the photo. I use a drive on the right system, and my main question is, why did this break everything? I was able to manually drive between stations, and I had about an hour of solid train performance prior to adding this lone signal. What did it do to the train system? I mainly want this question answered so I can better understand how the train pathfinding logic works, and so that I can better understand how to use signals.

[twepy]

The answer to your question is that by placing that one signal you made that section one way.

Signals serve two purposes, one is keeping trains from crashing into each other and the other is setting driving directions, a signal at one side of the track tells a automated train this is one way and trains will not go against a signal, if signals are placed at both sides of the track its considered two way. Back to your situation, because of that signal you placed that block is now one way and because trains dont have another way to get to the station you will get the infamous no path error. The train at the station however can leave the station because the one way allows it to find a path to the next station.

My suggestion would be to put a regular signal at the yellow spot and a chain signal at the green spot, then it two way again and the chain signal keeps a possible second train from entering and blocking the intersection. The difference between regular signal and a chain signal is that the first only check if the block its guarding is free or not, the latter checks if it can leave the chain signal block(s) if yes the its allowed to enter, if not the train is barred from entering the chain signal block.

The purple circle will cause trouble when the track gets busier, so my suggestion would be to change it to chain signal in front of the intersection and a regular signal after the intersection to keep the intersection clear.

[Selvek]

The easiest solution here is to move that signal after the rail split (near the roboport).

Also, looking at the right side where you have rails intersecting (crossing) each other, best practice is generally to have a chain signal directly before intersection and a regular signal directly after it. That prevents trains from stopping inside the intersection. For example, right now, imagine the green block has a train in it. A second train enters from the north, and makes its way clear to the signal separating the green and red blocks, where it gets stopped. Now, a third train is coming from the east on the white block, and is blocked by the second train.

If there were a chain signal directly before the north track crosses the red/white track, the second train would wait at the chain signal rather than entering the red block, and the train from the east isn't blocked.

Yay signals are fun!

[5thHorseman]

There is no signal on the northern track, above the roboport. So the game doesn't know it's one way.

[twepy]

There is signal, its above the power pole.

[besselfunction]

The answer to your question is that by placing that one signal you made that section one way.

Fascinating. That makes complete sense now that you mention it. I will try adding signals where you suggested. I will also add the recommended chain signals. I'm still a little fuzzy on the logic of chain signals. I understand conceptually what they do, but I need to work with them a little more in practice to really feel comfortable with them. I appreciate the swift replies from everyone!

[5thHorseman]

There is signal, its above the power pole.

Oh sorry. I (and Factiorio) misinterpreted your want based on the setup. The problem is actually the opposite of what I thought.

By placing the signal that you eventually removed, the section of track your train was on was turned into a one-way track. So, your train could not back up to the split.

EDIT: And I"ve been ninja'd by hours. I really need to start reading the whole post (and refreshing the page) before answering things