Factorio Forums

http://i.imgur.com/ki8nT9F.jpg

I want the marked signal to let through trains that are going from left to top when there is train going from right to bottom. Is there a way to do it or do i have to build it bigger to fit signals in the middle?

You would have to make it bigger. Since the middle has many crossing rails, it is all one block, and so only one train can use it at a time. You have to break it up into multiple blocks, but currently there is no room for signals there.

to achieve what you want you would need to do this. mark off the paths that the trains (one from left to top, and one from right to bottom) would take.

Now ANY piece of rail that is between their two paths has to have a signal on it (in the direction of travel of the rail). that means for example the two curves in the center that are for bottom to left and top to right travel, have to have a signal each between their crossings with the two rails you marked.
To get the effect you want, so that the signals actualy fit, you'll need to make this whole thing considerably larger (much more open space in it), and it will cut up the intersection into so many pieces that trains will constantly enter the intersection, block part of it, but not be able to continue cause a furhter piece of their path is red. and you'll get massive gridlocks on this intersection.
So until 0.12 (when it'll be possible to cut up an intersection like this but make it so a train waits for its whole path to be clear before entering the intersection), just dont try to do what you want.

The more signals you try the higher chances you have causing a deadlock. I think the best solution is just to face that trains will halt at crossings.

Unfortunately you can't do that with all signaling, you can't prioretize trains, thats handelt by the pathfinding.
If the pathfinding thinks this train goes first it goes, even if you wanted to let all trains that go East/west or North/south first, it's evenly possible that a east/south West/north train goes first.