Locations with both standard and narrow gauge track?

So, a question I've had for a while now about Sodor is how many places are there on the island where there's both standard and narrow gauge track. I mean, we've seen several locations where the narrow gauge engines can interact with the bigger engines, but how many of these places actually exist? Also, given that Skarloey was brought to Sodor during the construction of the original railway, doesn't that mean that there used to be a lot more narrow gauge track on the island then there is now? I mean, we know Duke's railway was all narrow gauge. Did that track get replaced by standard gauge track over the years? I just think it'd be interesting to know how the island was divided up over the decades and who decided what parts would be only be accessible by narrow gauge engines and what parts would only be used by standard gauge trains.