Train Signals: Do I really NEED a chain signal at the entrance of a split?

[tangofan]

Hi all,

In the image below I've shown two rows with possible signaling a train split and a train merge. Enough space is assumed before and after the split and the merge to allow for regular rail signals wherever possible.

Factorio Train Signal Merge+Split.jpg (1.41 MiB) Viewed 366 times

The top row shows how I usually place the signals for a split and a merge.

The bottom row shows a signal placement with fewer signals used, resulting in a bigger block before the split and after the merge. It is my understanding that the merge is signaled corrected. But what about the split? Can I just drop the chain signal before the split? It SEEMS possible, but I haven't found any definite confirmation on that.

Why would I want to do drop that chain signal before the split? Dropping that signal makes the preceding block a little longer and sometimes that extra length is enough that I can signal the entry of that preceding bock with a regular signal rather than a chain signal. Especially in the case of interchanges this may result in trains not being in the way of other trains.

But in order to reap that benefit, I'd first need to know, if the bottom row split is signaled correctly. Any insights on this are greatly appreciated.

[r3nt5ch3r]

No need, but the train will lose the option to chose the next free path. Instead it has to wait on the chosen path until it's free.
It depends on your setup and what you are doing in each path. Normally trains should take the bottom path, but sometimes trains are weird.

grafik.png (215.43 KiB) Viewed 343 times

This could be interesting: https://wiki.factorio.com/Railway/Train_path_finding

[tangofan]

No need, but the train will lose the option to chose the next free path. Instead it has to wait on the chosen path until it's free.
It depends on your setup and what you are doing in each path. Normally trains should take the bottom path, but sometimes trains are weird.
grafik.png

This could be interesting: https://wiki.factorio.com/Railway/Train_path_finding

Thank you so much for the quick answer and the link to the article on train path finding. It sounds then that in general I'll set the chain signal before the split, but I'll skip it, if it allows me to change the preceding signal from a chain signal to a standard signal.

[Kalanndok]

I'd put the rail signal at the split (not the one on the main line) directly after the split.
So only one train goes into the split and once it's in the mainline is free again.

Similar thing for the merge.
Keep the mainline signals at their proper distance
Then put the rail signal right behind the train stop (making the route from the train stop to the mainline effectively part of the mainline block).
If there is no train stop then put the signal on the merging branch right before the merge.

No chain signals at all.

[Kyralessa]

The point of a chain signal is to keep a train from blocking a shared section of track if it can't clear that track.

For example, if you have

station loop - shared single track - station loop

At each station loop you put a chain signal before entering the shared track, and a regular signal upon leaving the shared track. Then a train won't enter the shared track unless it can be assured of leaving it again at the end. If the regular signal block at the other end is occupied, your train won't enter the shared track, because then it would get stuck on the shared track and block every train. If every block is already full at the loop it's heading to, this will cause a deadlock.

Your case is a bit different, but I'd say it still needs a chain signal:

• Without a chain signal, your train could block the entrance to the two split tracks, preventing any other train from entering either one.
• But wait: If your train waits at the chain signal before the split, it's blocking any other train anyway, right?
• The difference: If your train enters the split and commits to one side of the split while blocking both, now even if the other side of the split clears, your train can't move and no other train can either.

So the point of a chain signal before the split is to keep your train from committing to a branch of the split before it knows which side has room.

(Actually this is the same thing r3nt5ch3r said much more concisely. )

[tangofan]

Your case is a bit different, but I'd say it still needs a chain signal:

Hi Kyralessa,

Thanks so much for your response and your explanations.

It seems that our definitions of "need" vs. "want" (aka "highly desirable) might be a bit different. My definition of "need" for signals contain only two things:

• Avoid train crashes
• Avoid train deadlocks

Everything else falls in the "want" category for me.

• Without a chain signal, your train could block the entrance to the two split tracks, preventing any other train from entering either one.
• But wait: If your train waits at the chain signal before the split, it's blocking any other train anyway, right?
• The difference: If your train enters the split and commits to one side of the split while blocking both, now even if the other side of the split clears, your train can't move and no other train can either.

So the point of a chain signal before the split is to keep your train from committing to a branch of the split before it knows which side has room.

The situation you describe falls in the category of "want"/highly desirable and in most circumstances I would indeed set a chain signal at the beginning of a split. But it doesn't appear to fall in my "need" category.

Furthermore in the situation I described, setting that signal might force me to guard the preceding block by a chain signal, rather than a standard signal. And that might keep trains from entering that block and if there is an intersection before that preceding block that might cause a train from even entering the junction. I agree that it is not cut and dry as to which approach is better in that situation, but to me it seems that it's perhaps better to get that train further and through that junction, even if it may mean that it already needs to commit to a branch of the split.

But reasonable minds might differ here. But what is important to me is that you also implicitly confirmed that omitting that chain signal before the split does not cause train crashes or deadlocks, but might be suboptimal in some situations. Thanks again for your response.

[coppercoil]

In your setup you need a chain signal before the split if you seek for the best performance. The train should wait before the split because it is not known which branch will be free first.