Edit Oct 2017: after a lot of revisions, I have a much better design with a Moving Knuckle now working:
This has significant advantage that the servo is not actively holding the knuckle closed.
I have also updated the main knuckle coupling geometry so that the 'other' couplings are now all single piece. Unfortunately the revised geometry of the coupling means that it is not fully compatible with this original hook design.
Edit 30 May 2017 .. Getting a bit fustrated with this design.. Since changing to push capable knuckles, I have realised that the servo is underpowered. It is not strong enough to actually open the couplings and release.. I am still working on the concept, but please do not think this design is 'proven' ..
EDIT 28 Feb 2017: Modified the knuckle to be "push capable", in line with my revised push capable knuckle coupler. http://www.thingiverse.com/thing:1989545
Use the new "Push capable" knuckle and tang STL's in place of the old ones.
This is the electric version of my LGB remix of Raby's Knuckle Coupling.
It uses a slightly modified version of the knuckle and tang, which have a small detent added to make the knuckle actively open when the servo drives back a wire armature.
The Armature is the most difficult part to make and is quite critical to the operation. I made mine of 1mm spring steel wire, which was difficult to bend precisely. I designed a small jig to make bending easier, and photos of how it was intended to be used are included.
Once bent, the key thing is to fit it, and check that it moves smoothly. It shoud be fitted to the "lowest" of the three holes on the servo arm, and the top of the servo arm will need to be cut off so it has enough room to move.
All parts will need some filing for smooth operation. The Motor Clap parts need to be glued (use acetone) to the Main Body. There is also a small knuckle part that needs to be glued to the bottom of the knuckle.
Assemble the tang first into the main housing, and then slide in the knuckle.
The assembly can then be fitted to the loco. The front coupling on the LGB Stainz has a deeper "front plate", and if you do not want to modify your model, you will have to use the "Stainz Front" variants of the motor clamp, Bottom housing and Main housing". This version has an extra cut out that allows it to fit, but you will have to do some filing to make sure the Tang and Knuckle can move freely.
Once you have the Armature nice and square (note: this is difficult) then fit it to the servo.
The servo then slides into the bottom housing, and the "front tabs" on the servo pcb should just be retained under some notches in the housing. The "back" of the servo, where the wires are, will sit down into a small recess. Bend the servo wires under the housing, and put some sticky tape on the. This will (should!) allow you to hold the box without the servo falling out.
Fit the main housing to the Loco, making sure that the knuckle and tang can move freely.
Then fit the "bottom housing", making sure that the Armature slides cleanly through the slots and is correctly engaged with the springs on the Tang and Knuckle. It is helpful at this stage if the servo is connected to a servo tester so you can check for free movement and operation.
Finally fit the two 1.7mm by 10mm screws and you should be finished.