UPDATE: After using this for a while I have noticed that setting the engaged and uncouple positions on the servos is very critical. If the servo is too far "engaged", (ie pulled "back" ) then the hook cannot spring down to couple easily. Also, if the Unlock position is too far "forward", then the hook on the carriage can get caught behind the front part of the pusher. To overcome this, I have modified one of my trains by removing the front of the pusher entirely. I then glued a small piece of the removed pusher onto the hook, to keep its geometry when engaged roughly the same. (I have added a new file "hook3" to show this). The original reason for the front part of the pusher was to provide a removable lip to keep the hook of the carriage engaged, but with the hook from the loco engaging in the carriage, there seems little point in this, so I removed it and will see how well it works on the track. It certainly decouples very easily and the "engaged" servo position now seems much less critical. Please send feedback if you have any improvements. video with full design working decoupling from a carriage (servos set correctly!) ... https://www.youtube.com/watch?v=UJGuc5AQuE4 The parts Loop2 Pusher2 and Base2 are very slight modifications on the original (still available in the 123D file) based on my building the first "production run".. I have found that it now works well with the same magnets in the loop and hook - so you can use 3 mm dia * 4 mm long magnets in both parts. This is a complete revision of my earlier (http://www.thingiverse.com/thing:398423) and http://www.thingiverse.com/thing:377751 couplings. The hook is now free to move in the horizontal axis, which should make the coupler work better around corners. I have also revised the servo's housing, and it now allows space for the wires connected at the bottom!. The new magnets hold the hook firmly in the centre , "engaged" position, but allow the hook to engage with another carriage when they are pushed together, and completely replace the difficult to adjust springs in previous versions. As the pusher pushes forwards, the pivot pushes forwards and tilts down, pulling the hook away from loop and de-coupling.. Make with 0.2 layer height, and 0.4 mm nozzle if possible. I have revised the space for the servo, but be very careful with swarf and blobs near the gears as there is very little room. I use a file to clear this out. The servo should fit with its front two legs under the triangular retainers. The rear is held in place with a 0.5 mm steel wire. The picture shows how this all fits. The sliding parts of the pusher and loop have been made so they can be easily cleaned out and smoothed with a 1.5 5 mm flat modellers file. Cut out the 8 mm hole in the "loop", as well as clearing out the slot for the connecting screw towards the back. (these were filled and have thin closures to help the underside print better) drill out the 1.8 mm clearance holes and the 0.5mm holes in the base. Drill out the 3mm holes for the magnets. The dimensions at the back upper part of the hook are critical.. If the back is too high, the decoupler will jam (the servo has very little force).. But if too low, the hook may drop right down to the track when decoupling and will derail the train as it catches the sleepers. I put three different versions in the hooks options for printing, but I found printing the one with a slight slope worked best, but filing to get best motion is probably essential. It was originally designed to so the magnets should just release the hook to drop under gravity as the pusher is slid forwards. However after building two with the slightly chamfered back to the hook I found it very nicely forces the hook down. But pre assembly, filing for smoothness and testing before fitting is essential. Assembly to the train is a fiddle especially locating the servo in the base properly, but i would recommend having the servo connected to a servo tester, so you can easily move the servo drive lug to align it with the hole in the pusher. Pre assembly and testing before fitting to the train is essential. When fitting the train, assemble the pusher in the loop, and connect to the loco using the original LGB coupling screw fitted through the slot hole in the loop. The squarish hole in the pusher allows you access to the screw. The LGB loco will probably have a 8 mm peg that fits in the hole you should have cleared in the pusher. Hold the whole thing together with 1.7 10mm pan head cross head screws (the hole in the base is sized to allow a 3.2 mm cross head screwdriver to fit these). If you forget the 8mm hole, the whole assembly will be angled down and will not work properly. Also be careful that the train centre buffer does not distort the main loop. My Stainz rear buffer slightly distorted the rear loop and stopped the pusher working. I filed the buffer to correct this. Following my tests I modified the files slightly to correct some slight issues, (2) There is now a second "loop" file with different mesh-mixer designed supports to make printing easier.