I've run cable in the gravel to supply power to three locations around the loop, as a belt-and-braces backup to the track joins - I'm running digital, so don't need complex wiring. I used some heavy-gauge hi-fi speaker wire from Maplins; I'm hoping it will last well enough, since it's neither exposed to the elements nor actually buried in the earth - I laid it in the trench bottom on top of a little gravel, then filled up the rest on top of it. Armoured cable or other very heavy stuff seemed a bit of overkill. At the conservatory, the three cable sets are piped through a conduit under the block paving, then up and through the conservatory wall to connect to the central station unit which will live inside the conservatory. Outside, no wiring will be visible at all except where a couple of inches poke through the gravel to connect to the track feeds.
Jon.