The SLRR right-of-way is open for the use of all Residents. Residents or groups with parcels adjacent to the SLRR tracks can request interchange trackage. We can't really guarantee that an interchange will be provided; approval will be based on terrain, prim availability, number of other interchanges in the area, and of course the nature and suitability of the Resident's build. Interchange tracks may be removed by the Linden staff if the Resident or group controlling the adjacent parcel don't seem to be entering into the spirit of the thing.
See the SLRR wiki entry here.