I cut the rail using a Dremmel drill with a cutting disk, unfortunately I only had quite a coarse disk so the gap is a bit huge but in tests it seems OK.
A bit of a blurry picture, the break is in the left rail
Because all the wiring is above the baseboard but under the scenery I needed to run the new wires this way too. I drilled two holes in the wall next to the shed and slid the wires in through them, The control panel doesn't have room for an extra switch and it would have been very fiddly trying to get the wiring to it so I decided that an extra switch mounted out of the way would have to do.
Grey wires before fixing down
The switch is mounted in a metal bracket which seemed just about the right size, I had to enlarge the hole for the switch but that was easy enough. Fortunately the original wiring which feeds this area is under the bridge so all I had to do was wire the switch across the gap to isolate a loco at the end of the headshunt.
The new switch
The whole fiddle yard and control panel
The last job was to disguise the new wires. I glued them alongside the track and ran them round the back of the green house and back to the holes in the wall. My favourite sandy paint stuff made the perfect material to bury the wires in, it needed a bit of touching in with some paint but did the trick. Where the wires go round the back I just glued some more bushes over them and Bob was my uncle.
It just needs a clean of the rails and a couple seconds with some paint and all will be well. Of course I now need to test the new section and have an operating session... well play trains.