Hi Paul,
I have built, what is effectively a scissors crossover, in EM and using DCC, on my layout, Elmham Market (although it is heavily disguised in the goods yard). Remember that DCC uses positive and negative in exactly the same way as DC, so to an extent that is a bit of a red herring.
I am sure there must be many ways of doing it but my solution was to pair the points so there is a master and slave for each route. I used Fulgurex point motors as they have two pairs of switches and are quite kind on the points, but Tortoise or DCC Cobalt motors work equally well (and I have been using Cobalts as the Fulgurex’s fail). Then isolate the frogs on the diamond crossing as if it were two live frog points back to back and wire it so the polarities are controlled by the polarity switches on the points that feed into it (you will need to sit down and draw your wiring diagram for the diamond so you know what frog you are wiring to which point as the polarities might seem a bit counter intuitive).
The diamond then becomes a second slave to the master point controlling the route. The next bit is critical. If both master points are directed towards the diamond you will get a short. The way around that, which I adopted, is to introduce a form of electrical interlocking (again using the extra switches on the point motors) so that if one master point is directed towards the diamond, the other is neutralised and cannot move (and vice versa). You don’t want to create a conflicting movement anyway, so this isn’t an issue. I hope that helps but very happy to try to explain better if all I have created is fog!
regards
Nigel
