Did you get any further with this Robin? While I was meandering I came across this:
http://www.aspenmodel.com/Frame_us.htm
Lovely semi-hand-made dual-gauge track in N/Nn3 (not to mention lovely code 40 track in N, Z, and Zm (!) too!). The price is $35 for a N/Nn3 semi-turnout (i.e. the narrow gauge goes one way and the standard straight through), which looks like what you need for that diagram if you don't want to turn to the dark side. They also have a stack of locos too in kit or RTR...
While I'm here, can someone explain if I'm missing something on dual gauge track? Obviously the narrow gauge loco can trundle down with its narrow gauge cars, and (separately) the standard gauge loco can go the same way with its standard gauge cars. But they can't do anything about each other, surely? The stuff would have had to have been unloaded and reloaded in real life, no? You can't even pull a narrow gauge car along dual gauge track with a standard gauge loco as the couplers would be misaligned. So what exactly is the point of dual gauge track, except to show off your handlaying abilities?
Charles