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A roundhouse and turntable is actually a very good little geometric puzzle for a high school drafting class. The length of the turntable would vary, as railroads only wanted a turntable long enough to turn the longest locomotives that would be assigned there, and the longest locomotive might have grown larger in the years after the turntable was built (such as when articulated steam locomotives became popular). I don't know whether turntable pits were enlarged over time, but I bet that some were. Anyways, the optimum length of the turntable bridge is just a little bit longer than the longest locomotive. Then, the optimum track spacing around the pit comes into play. You'd think that the tracks would just be placed at some regular angular interval, such as every 10 degrees, but really what was critical was the spacing of the tracks at the point that they enter the roundhouse, such that there is enough space between track centers to fit the doors, their hinges, and the structural column(s). Inside the roundhouse, the space between tracks opens up, creating workspace. If the angle between tracks is too great, then too much workspace is created, in other words space is wasted. If the angle is too small, then either there will be too little workspace, or the roundhouse will have to sit back away from the pit by a good distance. Use circles of varying diameter to lay it all out; once you get the right angular spacing, you can just copy the first track into a rotated array (I don't know what sort of CAD software you're using). The first circle is obviously the pit, at whatever diameter you've chosen. The next circle gets placed such that its center is on the rim of the pit. This circle would have a diameter of five feet, which is approximately the distance from the outside edge of one rail to the outside edge of the other (inside gauge is 4'-8 1/2"). Do another 5' circle centered on the intersection of the rim and the first 5' circle, and then where the second 5' circle intersects the rim will be the centerlines of the two adjacent tracks; the centerline of any track will radiate out perpendicular to the rim. By marching the 5' circle around the rim, you can place all your stall track centerlines. Incidentally, this is how we old fogies would do it using compasses instead of computers. To figure out where the front of the roundhouse is, you need to figure out how wide a typical stall doorway will be. You know you need clearance for the engine itself, which would ordinarily be about 16 feet wide. I don't know if roundhouse stall doors were tighter than this; they might have been, since any locomotive would be less than 11 feet wide. To this opening width, you need to add the actual widths of the door panels, since when they are open they will be parallel to the track, and then you need to add enough width for the structural supports inside. Guessing that the doors might each be 4 inches wide, and that the required structural width comes to around 16 inches, you come up with an additional 24 inches (2 feet), so that a typical stall width works out to 18 feet. Since you have track centerlines laid out, pick two adjacent ones, and offset (create a parallel line) one line that is 9 feet from each centerline. Where these two offset lines intersect, the width between the adjacent tracks is exactly 18 feet; the front of the roundhouse would have to be here or somewhere beyond this point. The roundhouse itself should be a bit deeper than the turntable bridge length (the rim diameter), at least in some of the stalls, so that the entire locomotive can fit inside. Sometimes roundhouses have varying stall depths, with a few shorter stalls just fit into a tight situation. Hope this helps, and I hope you have fun with the project!
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