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  1. Context The text within this post is based on an article I originally submitted in September 2014 to RailBricks for consideration to be included in a future issue of RailBricks. In the absence of any RailBricks issue since the article was submitted or recent communication from them, I have elected to post a modified version of the article to this forum. Introduction Figure 1 - Brick built Diamond Train Crossing 13 and 3. Two numbers which are key when building the diamond crossing shown in Figure 1 (the build instructions for which are available for download as a PDF document). Before explaining the numbers further though, a brief explanation of the reason for writing this article. It's not hard to imagine that one of the success factors in LEGO being played with worldwide is the absence of words in their build instructions. Build instructions, such as those for the diamond crossing, are intended to stand on their own without the need for additional commentary. The purpose of this article is to pass on the knowledge gained in the construction of the diamond crossing to other AFOLs. It contains information that is not appropriate to place in building instructions. The desire is to encourage others to experiment further to see what can be achieved in providing alternatives to the standard LEGO PF/IR train track parts. 13 and 3 Explained Figure 2 - Track Cross Section Figure 2 illustrates a cross section of a LEGO standard straight train track, 160 LDraw Units (LDUs) wide, complemented with a couple of extra plates. The image was generated using MLCad [1] and LPub [2]. Each rail has a 20 LDU footprint. The yellow plate was moved from its "correctly aligned" position on the partially obscured stud to its left, 7 LDUs to the right. The right hand side of the plate is touching the left hand side of the top of the rail. Since the rail parts are symmetrical, there are 7 LDUs of "space" on the right hand side of the rail, meaning the rail width at the top is 6 LDUs. Thus aligning a brick to the inside edge of a rail requires an offset of 7 LDUs + 6 LDUs i.e. 13 LDUs. Figure 3 - Solution Concept Figure 3 illustrates the concept of what needs to be built to achieve the same spacing as that between the two rails on a standard LEGO track part. Didier Enjary's advanced building guide [3] is a useful source of information about different techniques to obtain offsets. However, the offsets identified are even numbers, which won't combine to produce the desired offset of 13 LDUs. One of the answers posed to a question about how to achieve a 1 LDU offset [4] identified that there is a LEGO part that provides an odd valued LDU offset. The Minifig, Neck Bracket with Back Stud (42446) [5], as illustrated in Figure 4, produces a desirable odd value offset of 3 LDUs. Figure 4 - LDU Offset Bracket Built from inside to outside Figure 5 - 94 LDUs separation The crossing is built from the inside towards the outside. A fundamental principle was to create the equivalent gauge to a real track piece by obtaining a separation between bricks of 94 LDUs. Figure 3 illustrates where this LDU measurement comes into play. Figure 5 illustrates a combination of pieces from the crossing that achieves the desired spacing. Other brick combinations exist that provide the same separation. Potential for Re-use The diamond crossing is built, fundamentally, as a straight through track with a diagonal track attached to each side using a Technic angled connector. This approach yields a high potential for re-use of the straight through section if building a differently angled crossing, as follows. remove the diagonal tracks (see Figure 6) replace the angled Technic connector (highlighted in Figure 6) with one of a different angle for the hinge connectors' forming the internal diamond, alter their angle appropriately. This will also likely require a rework of the parts which form the diamond as a different length of track will be required for a different angle (see blue highlights in Figure 7). tweak the bricks used where the tracks join to adjust the length where necessary (see red highlights in Figure 7). Figure 6 - Straight Through Track Figure 7 - Adjustments required if angle changed Other Notes The crossing is built by creating a brick separation of 94 LDUs, working from the inside of the track to the outside of the track. At one point during construction, an alternative approach yielded an offset of 13 LDUs when combined with a baseplate, as illustrated in Figure 8. This approach was not explored further but does offer the potential for constructing a crossing on the basis of from the outside to the inside. Readers are encouraged to experiment to see whether a suitable crossing can be built using this approach. There is no guarantee that the approach will yield a usable crossing. Figure 8 - Candidate for an outside to inside build The crossing uses one part that is no longer in production; the "Hinge Plate 1 x 2 with 3 Fingers" [8]. It may be possible to use the "Plate, Modified 1 x 2 with Handle on End - Closed Ends" [9] instead. During the design evolution of the crossing, both parts were trialed in different versions. In the end, the decision was made that the profile (see figure 9) of the hinge plate provides a better functional fit than that of the modified plate. Figure 9 - Hinge Plate and Modified Plate Profile The key measurements 13 and 3, identified in this article, may be usable to build straight tracks suitable for use with the curved "narrow" rail track [7]. This has not been explored. Conclusion The primary purpose of this article has been to disseminate the knowledge gained about significant LDU measurements applicable when building a custom PF/IR train track diamond crossing. Armed with these measurements, the AFOL train community may be able to derive further track piece alternatives. References [1] Mike's LEGO CAD [2] LPUB [3] The Unofficial LEGO Advanced Building Techniques Guide. 2007, Didier Enjary. http://photos.freelu...B/SBv1.pdf.html [4] How to make a 1 LDU offset http://bricks.stacke...-a-1-ldu-offset [5] Catalog: Parts: Minifig, Body Wear: 42446 http://www.bricklink...tem.asp?P=42446 [6] Catalog: Parts: Technic, Plate: 4262 http://www.bricklink...Item.asp?P=4262 [7] Catalog: Parts: Train, Track: 85976 http://www.bricklink...tem.asp?P=85976 [8] Catalog: Parts: Hinge: 4275 http://www.bricklink...Item.asp?P=4275 [9] Catalog: Parts: Plate, Modified: 60478 http://www.bricklink...tem.asp?P=60478