Train Traffic jam [picture heavy]

[realm174]

Disclaimer: There is nothing wrong with the game, it's all me doing it wrong (as C.Will would say). And this is no sarcasm, I am aware of it, I just can't figure out a way to fix this.

I use LTN mod, but this also has nothing to do with the mod. It has to do with my laying down the rails I guess. But in a nutshell, I have stations that request various ores, and trains go and get them and bring them back. This is probably better explained in a picture. So here's what my original setup looked like (for the record, I've disabled most trains while writing this and taking screenshots, for the sake of being able to see the rails better):



In this scenario, trains merge in towards the inner unloading area from the left (or the bottom). They unload at the top area, and merge back into the system at the bottom. So you can, just by the layout of this, imagine where the bottleneck is: at the bottom, merging back into the network.

After trying a few things, I came up with this scenario:



Trains come in from the top left, go all the way south, east, north, then split into the different unloading tracks, and finally merge back up at the north end. In this setup, I actually experience a lot less traffic jams, but I still do. If you follow the inner track, you'll see that on the east side, it merges back with the outer system. I get some traffic jams there. The track also go further south, and then turns west, for the traffic going to the other area of the map. Maybe it'll help you see the whole picture:



So the first scenario above is on the south east side of this larger map, and the second one is on the north east area. So when trains leave the northern depot, they either either enter the network on the right, if they're heading south, and if not, will loop around the refineries in the middle, going west. My main traffic jam area there is where they merge to go west.

All together, there's about 40 trains going around in the whole thing. The entire map is much larger than this, and most of the mining area are on the outskirts. This central area is the main base, aka, where everything gets processed.

So what I get of all this is that my issue, regardless of which layouts I've tried, is when multiple trains go back into the system after unloading their goods. I understand that I have 8 lines merging onto 1, but can anyone suggest a different way of doing this so that I can get the traffic to go back out without stacking up at the merge points?

Much appreciated in advance..

[Koub]

There can be only one train in one rail segment at a given time.
A rail segment is defined by the set of rails that touch themselves and are surrounded by signals.

So if you have many trains that want to get into the same segment, they have to do it one after the other.
On the other hand, if you cut long lines of rail into smaller (but still longer than your trains), you'll be able to put more trains into the same rail line, thus allowing more trains to get through it in a given time.

[Engimage]

As Koub said the main question is about signals.
Generally speaking you need to manage the merging point with care
The simplest scenario for merging point is having a chain signal right in the end of every merged lane and a normal signal right after the merge and say every electric pole after that lihe here:

Common simple merge

Simple merge.jpg (172.44 KiB) Viewed 4272 times

This type of merge makes the whole exit area a single block effectively preventing any trains from exiting their lanes while any train is within that whole large block + 1 block after the normal signal. This makes a severe bottleneck on exit.
But you can make it a bit more complicated and avoid chain signals like this:

High throughput merge

High Throughput Merge.jpg (190.43 KiB) Viewed 4272 times

You do not actually need chain signals here as there is no possibility for deadlocks. You just need to prevent trains from crashing into one another and normal signals do that just fine. So for this purpose such design is the best.
Bare in mind that you absolutely NEED normal signals further along exit line every electric pole to allow trains following each other at a reasonably small distance.

[realm174]

Thanks gentlemen, that helped. Especially the high throughput merge. I had chain signals there, and when I replaced them with regular, that made things much better at that end. I guess there's not much else I can do, the reality of it being that I am merging 8 lanes into one, so there will be times where it backs up somewhat. My only alternative would be to split the area so that not all unloading stations are in one location and merge onto the same lane. That might be for the next project... (Factorio is awesome, there's always a "next project" in the pipeline)

[Engimage]

Glad I could help.
Regarding further improvements. If you still have a heavy jam on exit you might want to do 2 merges of 4 lanes to 1 and have 2 exit lanes which exit to your global system in 2 different spots.
However if your exit line can't handle the traffic it means that no single lane can do it and you have to expand your rail system to 4 lanes around this station. It is a good idea to split iron from copper to use different set of lanes. Say Iron mostly uses inner rails and copper for outer. You can still have lane switchers to allow mixed routes but you can always try designing stuff the way I described. This will definitely increase overall throughput.

[vanatteveldt]

Just for information, the chain signals on the 'normal' merge can safely be replaced by normal signals. A chain signal prevents a train from stopping on a junction so other trains can pass, but since all trains are leaving in the same direction, they are all waiting for the same stretch of track to clear. So a merge can never cause a deadlock (by itself). You only need chain signals to keep an x-crossing clear, i.e. where it's possible for two trains to come from different directions AND go to different directions.

[ For a car analogy, it's often illegal to "block the box" on an intersection, so a cross street is not blocked by a traffic jam on the other street (essentially a chain signal: you can only enter the box if there's room to exit it), but in a highway merge you are always allowed to drive onto the merge lanes. ]

[realm174]

I like the analogy,it gives me a new way to think about signals when I lay them down, so I can decide if I actually need a chain signal, or if I'm just putting it down because of the so called rule "chain before intersection". Thank you for that.

Using a combination of tips from here and experimenting, I've cleared most of my traffic jams. For the newer train users reading this (or the seasoned one that might have forgotten) the more intersections you have, the more chances of trains having to slow down/stop, adding to the traffic jam issue. So after fixing my signals, and removing unnecessary junctions in a few areas, I'm happy to say that I've fixed maybe 90% of it. I just have to accept the idea that some areas WILL be a bit of an issue at times, and live with that.