If by interchange, you mean dual gauge track, I'd look at getting some O scale light rail, and using that for the siding, and the narrow gauge rails. Typically, sidings are lighter rail than mainline.
Unless you have dual gauge track, you can leave the standard gauge as is, and use the code 100 for the narrow gauge.
An interchange, or transshipment point can be a freighthouse with a standard gauge siding on one side, and narrow on the other, or as simple as standard and narrow side by side ( loads like ore, and coal were sometimes hand shoveled from one car to the other ). In most cases the "interchange" is designed to handle a limited variety of product.
The labor intensive bottleneck of transshipment was one of the contributing factors in the eventual phaseout of narrow gauge.