I'm new to the Lego train scene, although I've built up a decent collection of 9V sets over the past few years. This is my first real attempt at a MOC.
This gearbox connects directly to a standard 9V track switch (or set of points). Although the tabs for the yellow mounting lever are broken on this particular unit, the design still fits with those tabs in place. I've tested this on three different switches and it works fine. You DON'T need to open, cut or damage the points in any way for this to work. Instead, the Technic rack (3743) drops straight onto the points' sliding mechanism.
The old Technic 9V motor (43362 or 71427) sits on top, with the help of a couple of plates.
On the other side, a clutch protects the motor from stalling once the points have switched. I've tried this design with both the high- and low-torque clutches (light grey and dark grey centres respectively) and the points still switch and the unit stays in one piece.
Overall it's a very compact design. The footprint is 8 x 5 studs, but the protruding gears and Technic axle bring it to 8 x 8. However, it's really only two studs longer the existing point mechanism.
I wanted a design that used the older Technic motors because they're smaller than the newer stuff, very torquey, still relatively cheap and correct for the period (9V stuff).
As a small development, it's also possible to incorporate a gravity-driven signal mechanism into this design. Pay attention to the height of the white brick in these two pics:
In this design, the 1 x 1 x 5 brick (2453) has a 30 slope (54200) on the tip. The brick is held loosely over the end of the rack, so that it is held upright when the points are set straight, and then drops down when the tracks switch to the side. There's a lot of potential here for a "realistic" signal mechanism, but I don't have the spare bricks to explore this option yet.
Points in action: