They are easy to do with continous stock rails. I cut the stock rails so they will be exactly (or longer than) the length needed for the complete crossover, scribe the base dead center on both pieces, then place one in the fixture. Align the scribe mark precisely with the "center" end of the fixture, mark for the stock rail cut, remove it and place it on the other side of the fixture aligning the scribe mark precisely. Mark the second stock rail cut location. Do the other stock rail marking, cut the 4 places for the points.
I then build the first half of the crossover, including the full length outside rails. Make sure the scribe marks are perfectly aligned with the fixture "center" end. Then build the other half center portion, except for placing the point rails. Then align and support the first half and solder the outside rails in place. Finally, install the point rails. Add the center PC tie as normal after you remove the completed crossover from the fixture. Care must be taken to true-up the ends and de-burr them before soldering the first half to the second half. Fine tune for a perfect fit. A finely adjusted support for the first half while soldering it to the second half is a must.
If you make the stock rails longer than needed they can be trimmed during installation on the layout.
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