Datenails were tagging devices utilized by railroads to visually identify the age of a railroad tie. Different railroads used different sized nails with either alpha or numerical markings. An example would be a Southern Pacific Railroad nail with the marking "01" stamped on the head of the nail. The "01" would identify the nail as being hammered into a railroad tie in the year 1901.
Datenail use has dropped dramatically since the mid-20th century and the advent of more modern maintenance of way equipment. Ties are no longer marked in this manner in North American practice, and the nails themselves are now sought after by railroadiana collectors. The Southern Railway never made use of datenails.
Datenails are also found on utility poles, sometimes in conjunction with a nail showing the height of the pole in feet. The types of nails may have distinguishing characteristics, such as the datenail having raised digits and the "height nail" having incised digits. The pole height will be a multiple of five (e.g., "35" or "40"). http://nailhunter.com/