I have been continuing with the wiring of Glenmutchkin, but have hit a snag; one that I should have been ready for – the wiring of the slip,  I had been aware that a diamond crossing was a challenge to wire and I was suckered into thinking that the switches on a slip could over come the challenge,  Well I go that wrong…….!!

The basic problem is that there are a choice of two routes through a diamond crossing and each route requires the polarity of the crossings to be different.  The diagram below, which shows how a diamond crossing needs to be wired, should illustrate the problem.  The only solution to this is to power the crossing polarity by way of an approach turnout – if you really don’t have one to set the polarity with, then you are going to have to resort to some switches – but at least it will give you a good excuse to interlock the diamond crossing with some signals to remind you on which direction it is set!

Hopefully this is clear that the crossings on the diamond crossing are activated by detecting the direction of the switch on the approach turnout.  If it is set for straight ahead, then a train can’t travel over the crossing and therefore the parallel line can so the polarity of the crossings are set accordingly.  Conversely, when the approach turnout is set to the branch, the line across the diamond can be used and the polarity is set to suit.

The principal with the diamond crossing needs to be heeded when the crossing is replaced with a single slip as I have, but it does get more complicated because the switch of the slip can also lead to a different route through the crossings.  The crossing to the left of the slip is the more straight forward as it is only activated by the approach turnout.  However the right hand crossing is more complicated as if the approach turnout is set for the branch then it always needs to be in the red polarity whereas if the approach turnout is set for the main, then it then needs to be controlled by the slips switch.

Hopefully the diagram above shows how this works.

The irritation I have, in addition to having wired it up wrong already (!) is that the approach turnout is on a different board to the slip.  To reduce the number of wires crossing the boards, I have decided to simply use a duplicate point motor for the approach turnout located on the same board as the slip.  It is expensive but rather more simple than the additional wires.

NB – please see also a follow up post on this wiring arrangement for an alternative approach.