Ok, lets begin with the "Y" Junction

Step 1: Create a cylinder, doesnt matter what size aslong as its not too small for hammer, I made mine 32l, 32w, 156h, 16 sides:



Step 2: Duplicate that cylinder, and make it horizontal to the original with one end sticking half way into the original:



Step 3: Now using the clipping tool, cut the horizontal cylinder vertical along the inside edge of the original (make the outside of that cylinder invisible)



Step 4: now you will need to cut the visible part of the horizontal cylinder into pieces (half the number of sides)



Step 5: And lastly you will need to take each piece and morph it to fit the edge of the original cylinder:



Now, the final product: "Y" Junction:



Now you Should all have realized by now how to make the "X" Junction, but for those of you who havent, it's real simple.

Step 1: after completing the above steps for the "Y" Junction, simply double up the final product. You could even change it up a bit, add some style to the design.



Final Note: Do you, and your players a favor, use this wisely, and func_wall all the pieces to help your r_speeds (see various tutorials for an explanation)