Trail Lines are typically entered by tracing satellite imagery to extract latitude/longitude coordinates. The accuracy of these coordinates changes depending on the source of the photo and the precision of placement. I have seem satellite edges mismatched on the order 55 feet. Line work is sometimes extracted from the Open Street Map Project and from Strava heat maps.
>Milepost Known milepost (are placed on the map when they are visible in the satellite image or can be located via to visible features in the image. Other miles posts are extrapolated using calculations along the rail line. Note Railroads have also been known to realign trackage and not move existing mileage markers, only making an annotation on their paper maps.
Distances Above are based on the google map distance between two points along the line work.