Yup the unwanted uncoupling when used in a siding would be a problem and could only really be over come by lowering them into the baseboard when not wanted. As a hump yard I would want it to work every time so not an issue.
Caiptean: yes the speed could be an issue, I think I would only aim to use a 3 way point and live with being able to sort into three sidings any more and I think you would need the central ones arriving at speed and the outer ones creeping in. However as for a vague idea of slowing them I did consider some more magnets, this time pulling on to the wagon as opposed to repel it.
Steve: thank you for the origin in this case the re coupling is achieved by the fact it is on a slight incline before it reaches the bigger incline (to accelerate to reach the sidings) but the opposing magnetic fields means that the tension comes off as the wagon is pushed over the magnets even though its going down hill