"SR" Milepost Designations on Southern Tier Line

(I feel sure this may have been answered in the past somewhere but I can't find it)

Does anyone know why NS uses the "SR" prefix (and what it stands for) for the mileposts on the Southern Tier Line?

Thanks

It's just the appreviation NS uses for the Southern Tier line. Each line has its own. The Buffalo line MP's were BR for example.