Sorry, I should have posted a more newbie-friendly explanation with that.
If you don't already have it, download the AJS Traffic Controller (<kuid2:122285:1159:3>) and MiZi's Junction Link (<kuid:122381:10003>) from the DLS.
Locate the road spline of your choice in Content Manager, and open it for edit in Explorer. Open the config.txt file, find the line that says "istrack 0", and change it to "istrack 1". Save the file, then commit the asset in Content Manager.
Note that the road spline you modified will now be located in the Track tab, instead of the usual Objects -> Splines tab, as Trainz now considers the road spline to also be a track spline.
In the below screenshot (click the thumbnail to see the full-size image), I have placed a Y-junction using a road spline modified as described above, and placed an invisible, silent junction lever (I use "Strab Weiche unsichtbar", <kuid2:69084:38023:2>). Cars coming from the bottom-left of the screen will turn in the direction the junction is set to.
In the above screenshot, I've also placed an AJS traffic controller (the stretch of invisible track with the big red arrow on it). Place a short stretch of invisible track (here I used "AJS Track Eraser" (<kuid2:122285:1003:1>), the same track the Traffic Controller uses) with a Y-junction from the
opposite end of the Traffic Controller from the one the red arrow is pointing at, as seen in the below screenshot, and place an invisible junction lever on it.
The Traffic Controller will automatically flip the junction lever it is connected to according to the intervals set in its Properties dialog. Click the question-mark ("?") button in the Objects tab and click on the Controller to open its Properties dialog. Set both of the phase timing settings to 1 second. This will cause the Controller to flip its junction lever every second. If you want the Controller to leave the junction set to one direction longer than the other (to make it more likely that cars will turn in one direction than the other), set one of the phase settings to a higher number of seconds.
Now we need to link the Controller's junction to the road junction. Place a Junction Link (the little red cube between the two junctions in the above screenshot), open its Properties dialog, fill in the Junction 1 and Junction 2 slots with the two junctions, and you now have a road junction where cars will turn in both directions.
Hope this helps,
Zachary.