Rail intersection - feedback please

[zOldBulldog]

After a few attempts with rails I finally came across the concept of chunk-aligned blueprints. Life became much easier.

But the 4-way intersection seemed like it would be easily jammed. So I mostly stuck to T-intersections but every now and then I need a 4-way intersection and came up with this:

- Designed for 1-2 trains so that they fully fit in the straight and turnaround curve sections.
- The goal is to maximize train throughput for these small trains.
- Centered on a chunk, with each lobe occupying one chunk. The other chunk-aligned blueprints directly connect to it from the 4 directions.

Screenshot:
(please ignore the branch-off at the top right, it is not part of the design)

wideCross4way.jpg (295.43 KiB) Viewed 5259 times

Blueprint:

Code: Select all

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This is my first rail design so I am sure that it has flaws I could improve/address. I would appreciate feedback from experts, keeping in mind that I am intentionally only trying to address the following scenario:

- 2 lanes, right hand drive.
- 1 locomotive, 2 wagon trains.
- I will only use these on the few mainline intersections. For most other intersections I plan to use T-intersections or single-line one-way branch-offs, to limit impact of the "almost roundabout" design on pathfinding calculations.
- I do hope to support heavy traffic.
- The train network that will use this design will be relatively small - covering the nearby area around the 1st rockte main base. The megabase will be completely separate on its own train network, and any long-distance shipping will be on a 3rd network of very large trains. I don't currently plan to connect the 3 train networks to each other, but rather to use transshipment stations to transfer goods.

Edits:
- Updated the screenshot to show the rail segments.
- Link to the Bible of all 4-way intersections by aaargha: viewtopic.php?f=194&t=46855 (I wish I had seen this before spending time designing my intersection, there are some very nice - and tested - designs there)

[Hedning1390]

What's the point with having it chunk aligned?

There's a lot of conflict points in this, and by spreading them out you increase the risk of multiple conflicts per train. I don't see the benefit.

[zOldBulldog]

What's the point with having it chunk aligned?

There's a lot of conflict points in this, and by spreading them out you increase the risk of multiple conflicts per train. I don't see the benefit.

By chunk aligning you make it very easy to place track from blueprint.

[Deadlock989]

I wouldn't say I was a rail expert, but I've built a ~1 RPM base supplied by rail, and all this stuff is kind of subjective and personal, but ... the above looks huge and overdesigned to me. I just use this, I know it's simple and there are those who will scoff at it, but it works for me. I never paid attention to roundabout doomsayers and the base ran at 90-120 UPS with 54 trains on the network, with no isolation between base and outposts.

(Left handed - I'm British dontcherknow.)

[zOldBulldog]

LOL, deadlock... your design is exactly what I initially wanted to build, simple, small, and sure... it will only allow one train through the whole roundabout at a time, but that didn't seem to bad to me.

Then I read all that doomsaying and objections to roundabouts... and I first tried a compact 4-way that in my mind did the same as what you have but it looked to me as being even worse from the perspective of congestion. So I bit the bullet and went for what I designed.

But it is good to hear that your roundabout didn't cause performance problems. I will give mine a try for a while, but I might just end up using your design instead.

[Zavian]

I agree with Deadlock989. I'm also not a rail expect, but I've never had a problem with roundabouts. (Admittedly I don't disable station whilst trains might be enroute).

For a large collection of rail junctions see viewtopic.php?f=194&t=46855 .

[Deadlock989]

I made this sped-up video to show the base running to someone who thought that railway pathfinding is "broken", takes a while to get going properly from a standing start: https://youtu.be/__S-qmcQK-U

[Hedning1390]

What's the point with having it chunk aligned?

There's a lot of conflict points in this, and by spreading them out you increase the risk of multiple conflicts per train. I don't see the benefit.

By chunk aligning you make it very easy to place track from blueprint.

I still don't understand. How does that make things easier. What's wrong with something simpler, like any of these?:

[zOldBulldog]

@deadlock: Nice. I think you proved that roundabouts can be used, at least in moderation which is what I need.

@hedning: The one on the left looks (to my untrained eye) like it could suffer when there are a lot of trains. The one on the right looks like it might flow well, but I never saw it before... I might have used it if I had found it on factorioprints.

Even so, I don't understand what is your objection to mine, it seems to me roughly the same size and perhaps even simpler in design. What exact elements of the design are wrong? I don't think you've mentioned that yet.

[Hedning1390]

I said there are lots of conflict points, ie points where two trains contest over the same block. If I count a left turn you have 4 mergers and 2 crossings (because of the detour crossing its own line can slow down the train behind, so it still counts). Those conflict points are also very spread out meaning it will take a long time for a train to clear them all. You basically have a large roundabout, so it will function like a roundabout. A train making a left turn will only have 1 route untouched.

In contrast in my picture the intersection on the left is keeping conflict points as small and close together as possible while still allowing up to 4 trains to go through the intersection simultaneously. For comparison a left turn will leave 3 routes open, and has 2 crossings and 1 merger, but more important is how close together they all are, making them essentially one and the same. The intersection on the right is a diverging diamond interchange inspired intersection. It provides great options for trains on the main line (horizontal), including u-turns, but at some cost.

[Deadlock989]

it will only allow one train through the whole roundabout at a time, but that didn't seem to bad to me.

Not sure about that. In the pic, that train is heading from north to south. You can see the north exit is green and the west entrance chain signal is blue rather than red (not all the exits are blocked), so something should be able to go from west to north at the same time something else is going north to anywhere else.

[zOldBulldog]

Something you might have not noticed is that the conflict points are the corners and are pretty small, and because of the spacing the 1-2 trains I use fit completely in the segment between conflict points without blocking the next segment.

Also, the layout should normally keep trains on separate paths and not often encounter each other at a conflict point..

I would think that would reduce congestion.

I am not at the computer but perhaps tomorrow I can upload a screenshot that shows the track segments in color.

[Hedning1390]

I understand that you can fit multiple trains in that intersection because the intersection is large compared to the trains. But that means you'll instead have more encounters because each train spends a lot of time in the intersection. Especially since you don't seem to have regular signals that much in the middle of the interchange (maybe you do, it's hard to see in the picture). Every merger is a bottleneck where two lines turn into one. In car traffic mergers are much better than crossings, but this knowledge is treacherous when you try to apply it to factorio.

Any intersection can be enlarged by simply increasing every distance. It doesn't really help unless you use the space to create more lanes. Like one way your intersection could be improved very easily is to have right turns never enter the roundabout, but have completely separate lanes. Alternatively your interchange is large enough that you could probably fit a whirlpool interchange in the same space. That is a pretty good interchange for trains.

[Zavian]

it will only allow one train through the whole roundabout at a time, but that didn't seem to bad to me.

Not sure about that. In the pic, that train is heading from north to south. You can see the north exit is green and the west entrance chain signal is blue rather than red (not all the exits are blocked), so something should be able to go from west to north at the same time something else is going north to anywhere else.

Actually from the signals that train is heading from north to west. Two or more trains on non-intersecting routes should be able to use a roundabout at the same time. But zOldBulldog is also right in that really high throughput junctions tend to have dedicated line, and internal buffers for trains. (They also tend to be huge, because of those buffers).) The thread I linked earlier has screenshots and throughput numbers for a number of different junctions.

[Koub]

Just so that you know, there has been a very extensive rail intersection study made by our train expert aaargha :
viewtopic.php?f=194&t=46855

[dood]

Do buffers in an intersection really make things that much faster?
Seems more like a recipe for a deadlock to me.
Why not buffer the trains on the straight rail segments by spacing the signals 1 train length apart instead?

[Koub]

In a cross intersection (for example) you can have trains going from any 4 inputs to any 3 outputs.
Buffers in the intersections means a train has to wait only if its path would cross the path of another, but if there is no risk of collision, you can have several trains going from several inputs into several outputs at the same time. If one pair of "input/output" lanes is busy, and you buffer only for it, then all the others are still free to let trains pass.

If the "buffering" is made on the track before the intersection, then every incoming train will have to wait the intersection is clear before going in, blocking all the trains that are behind it (even if they would have been able to get through the intersection without waiting).

[Zavian]

Buffers won't cause a deadlock, provided they are big enough for your longest train, and signaled correctly.

For reference aaargha (viewtopic.php?f=194&t=46855) measured a roundabout similar to the one pictured earlier at 30 trains per minute (and he does not consider that roundabout to be deadlock safe. I think it could deadlock if something caused a train to repath in the middle of the junction).

Here is a medium sized 2 lane junction from aaargha's thread. Aaargha measured this one at 58 trains per minute, and he does rate this one as deadlock proof. ("Cross" design by Tallinu. Variant pictured is for 6 car trains. Note that even if something did cause a train to repath in the middle of the junction, it will always exit the junction via the route it has already reserved).


He also has 2 track variants that can cope with upto 90 trains per minute, (They are larger with even buffers); as well as 4 lane, 6 lane, and 8 lane junctions. (The fastest junction is the 8 lane "Multi-Cross" also by Tallinu which was measured at 352 trains per minute. http://i.imgur.com/oGHjCXM.png ). Blueprint strings and images are available from the spoilers in the first post of aaargha's thread.

[zOldBulldog]

Thanks for the references to aaarga's thread, that is clearly the Bible for intersections and exactly what I needed.

[dragontamer5788]

(and he does not consider that roundabout to be deadlock safe

Note that the deadlock testing was in 0.15. Some version between then and now helped get rid of deadlocks. So a lot of "previously unsafe" intersections became safe in the 0.16 versions (somewhere...)