There are a number of different methods for designing railroad interchanges. This guide describes how I build roundabouts in Satisfactory. Typical roundabouts have between 1 and 4 pairs of entrance/exits. The roundabout I'm describing also lets you add 2 more (with some limitations) for a total of 6 pairs.

Although roundabouts are nice interchanges, they have some disadvantages:

• Trains will slow down a little going through them because they have to make relatively sharp turns
  
• Even with proper signaling, trains might have to wait while the roundabout is busy, depending on where they want to go
  
• They take a while to construct

But I like them nonetheless:

• They inherently provide U-turns
  
• You can construct them with all 4 entrances/exits stubbed out without being connected to any further tracks and then expand the tracks in unused directions when you're ready
  
• They look cool

There are some limitations and behaviors Satisfactory has with its train system that I kept in mind with this approach:

• Track segments have a minimum length of 12m (at the time I wrote this guide), so the overall size of the roundabout can only be compressed so far.
  
• There can be no more than 3 track segments with a common edge: 2 inputs & 1 output, 1 input & 2 outputs. You can't have 4 segments sharing a common edge -- when you try to connect a 4th segment, you'll get a "too close to another switch" error. (In earlier versions of the game you could have 4 segments sharing a common edge, with 2 inputs and 2 outputs)
  
• The roundabout circuit is segmented into its own "track block" using path and block signals: each entry to the circuit is marked with a path signal; each exit from the circuit is marked with a block signal. This allows multiple trains to occupy the circuit at the same time, as long as their routes won't intersect.

Here is an example of a completed roundabout with the 4 standard entrance/exit pairs. It has a central support tower with some floodlights at the top and struts supporting the roundabout itself. One of the key characteristics of this design is that the struts attach to both the tower and the roundabout's platform at single foundation blocks without the need to do any offsetting:

The method I'm describing in this guide to make the roundabout is my own, but I have been inspired by a number of content creators and wanted to acknowledge them:

Amelie of the Sea: https://www.youtube.com/c/AmelieoftheSea/featured
DanP: https://www.youtube.com/channel/UCfKIEix6Gl--xv25svh0RjA/featured
Dekoba: https://www.youtube.com/c/Dekoba
I'm Kibitz: https://www.youtube.com/c/ImKibitz
Lukas Bilek: https://www.youtube.com/channel/UClFnLlX4c_1-BIuorLyH3Gw
Nilaus: https://www.youtube.com/c/Nilaus
Parkinwad: https://www.youtube.com/c/Parkinwad
Samueras: https://www.youtube.com/c/Samueras/featured
TotalXclipse: https://www.youtube.com/c/TotalXclipse

The first part of construction is to make an 8-spoke wheel with a single foundation block as the center. This center foundation block will also be the center of the roundabout. If you look at the image of the completed roundabout in the introduction, you will see the light towers placed at the edges of this center foundation block pointing at the entrances/exits to the roundabout circuit.

Step 1: Start with a platform. The roundabout supporting structure will be built on top of this platform. The platform is to make it easier to move around while assembling the roundabout and to make it easier to see what’s happening in these instructions. You can actually skip the platform if you use other methods (like the hover pack) to get around easily.

The roundabout circuit will be approximately 7x7 foundation squares in size, and you'll need another 2 foundation squares for each entrance/exit. so the overall area you'll need is 11x11. Creating a platform will help you determine if you have enough space to make the roundabout based upon where you want the entrances and exits:

Step 2: Place a foundation block where you want the center of the roundabout to be. The edges will be aligned with where you will have the roundabout's entrances and exits:

Step 3: Extend the foundation block 3 additional blocks in each direction:

Step 4: Place another foundation block on top of the center block, rotate it 45⁰, and extend it 3 foundation blocks in each direction:

Step 5: Replicate the blocks added in the previous step so they are on the same level as the others. Then remove any unnecessary block(s) so you have an 8-spoke wheel:

Now we'll place the diagonal parts of the circuit (the segments on the circuit connecting the entrance/exit areas).

Step 6: For each of the diagonal spokes, add a foundation block on either side of the outermost foundation block:

Note the position of the power poles. These will mark the endpoints of the diagonal track segments in the circuit:

If you examine all the positions a power pole can be placed on a foundation block, there are 81: 9 in each direction (including right along the edges). The power pole on the left is 1 position to the right and 1 position outward from the center of the foundation block it is on. The power pole on the right is 1 position to the left and 1 position outward from the center of the foundation block it is on.

Step 7: Create a track segment between the two power poles' positions with center of the track segment at the positions marked by the power poles (you don't actually place the power poles, they are there just to clearly indicate the locations to use):

Step 8: Repeat for all diagonal spokes:

Now we add the curved track segments to complete the circuit.

The circuit has 4 straight sides (the diagonals placed in the previous section) and 4 curved sides that will be placed in this section. Each of the curved sides is made from two segments whose centers meet at the center of the entrance/exit spoke as shown by the location of the power pole:

Step 9: Place a track segment with one end connecting to a diagonal segment, and the other end with its center at the location of the power pole. Start the segment at the connection point to the diagonal and end it at the location of the power pole so the track points in the right direction:


** NOTE **: Sometimes there are glitches that don't let you place tracks where they should be able to go. If you can't place the track segment in this step for some reason, you can [usually] do it a different way:

Step 9-a: Place two temporary foundation blocks at the end of the spoke opposite the diagonal segment you are trying to connect to. Note the location of the power pole on these temporary blocks:

Step 9-b: Now create a temporary track segment with centers at the locations of the power poles:

Step 9-c: You should now be able to connect the diagonal and temporary segments:

Step 9-d: Remove the temporary foundation blocks and temporary segment.

** I've run into situations where even this alternate method didn't work. I finally got the game to cooperate by completing as much of the roundabout circuit as possible (through Step 11), and then removing and replacing segments in a random or [counter]clockwise order to see if that would let me add the ones that were giving me trouble. If this still doesn't help, I'm sorry...the game is just being finicky.

Step 10: Repeat for all entrances/exits:

Step 11: Add the remaining segments to complete the circuit:

Now that the circuit is completed, we need to add the entrances and exits. This guide places the entrance/exit tracks with centers on the exact edges of the foundation blocks.

Step 12: Extend the spokes corresponding to the entrances and exits by 4 foundation blocks:

Step 13: Place a track segment on each of the outside edges of the 2 outermost foundation blocks at the entrances/exits. The segment should span the full length of these 2 blocks. These segments are essentially temporary, but they are needed so that the actual entrance/exit segments from the circuit point in the right direction:

Step 14: Repeat for all entrances/exits:

Step 15: Connect the entrance and exit segments to the diagonal segments in the roundabout circuit:

Now that the roundabout track is completed, we need to add the signals so trains flow through it properly.

If you begin to place a Signal you can see there is a single "track block" (all red):
We need to add Path/Block signals so that the roundabout circuit is a single "track block" and the entrances and exits are different blocks. All the entrances to the circuit will be delimited by Path Signals and all the exits will be delimited by Block Signals. This will define the circuit as a single "track block" and multiple trains can occupy that block at the same time, providing their paths won't intersect.

Step 16: Place Path Signals at all the entrances to the circuit (this example is for right-hand traffic):

Step 17: Place Block signals at all the exits from the circuit:

Step 18: Extend the entrance/exit spokes and add additional track segments so you can add an additional Block Signal at each entrance/exit (8 in total). This will simulate the tracks connecting to the roundabout to allow you to see the "track blocks" when trying to place a signal. Note that the circuit is a single "track block" (it's a single color) and all the entrances/exits are different "track blocks" (they are all different colors from the circuit):

This roundabout design allows for two additional entrances and exits. This can be very handy, but there are some additional things to keep in mind if you want to do this.

If you associate compass directions with the entrances/exits, the standard design places them at N, E, S and W. You can add up to 2 more at NE, SE, SW, and NW. But you need to be careful about the behavior of the trains when you do this.

If you add only one additional entrance/exit, it doesn't matter where you put it. If you add both, they need to be opposite each other (this is due to the limitation that no more than 3 track segment edges can be joined together).

If you have a train that is entering the roundabout and then exiting at the next 45-degree spoke (for example, enters at E and exits at NE), the path through the roundabout crosses over itself. This will not be apparent from looking at the spokes. You have to look at where the entrances/exits are able to join to the circuit (again, due to the 3 track segment edge joining limit).

The roundabout circuit will hold 10 cars (including locomotives), so as long as the longest train that is doing this has 10 or fewer cars, it's not a problem. Or if the train is exiting the roundabout at a spoke that is 90 degrees away or more, it's also not a problem. The problem only happens when both:

• The train is more than 10 cars long (including locomotives), and
  
• The train is going to exit the roundabout at the first possible exit and that exit is 45-degrees from where it enters

To add one (or both) of the additional entrance/exit lines, extend a diagonal spoke a few foundation blocks and place a pair of track segments at the edges a few blocks away from the circuit (just like what was done for the primary 4 entrance/exits), connect them to the circuit, and add the signals in the same way as was done for the others, as shown:

If you check the "track block" identification, you can see that the circuit is still isolated from all the entrances/exits with the addition of the 2 additional entrance/exit pairs:

Depending upon your needs for entrances/exits, you might want to rotate the roundabout. The only reason that might not work is if there are some glitches that prevent track segments from being placed (see the "Building Limitations" section of this wiki page[satisfactory.fandom.com] for some information about that).

So that none of my trains gets hung up in the roundabout, I make the "track block" right after each exit long enough to hold the longest train that will be going along that route. This way, once that train enters the roundabout, it is guaranteed to fully exit (providing the route it's following isn't backed up for some other reason).

Similarly, I make a "track block" preceding each entrance to the roundabout so that if the roundabout is busy, a train can still approach as close as possible while waiting for it to clear. If the block preceding the roundabout is very long and is occupied, the train behind the one that occupies it might be a long distance away.

Thank you for taking the time to look at this guide. If it has been helpful to you, you may find my other guides useful:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2836897112
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2842159512
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3346733784
If people are finding these guides useful, I'll try to find opportunities to make more of them.